SpellsAs the Sohei spells and their updates are so scattered over the books, I will list the most current source even if it is the PHB.
A cheap way to expand your tiny spell list that costs you nothing but locking you into good alignment are
Sanctified SpellsBoED. I will denote them with an asterisk (*) after the regular spells.
1stAttraction:OA Increases damage against a single target for a short while, and depends on your caster level. Target also gets a save against your mediocre DC.
Bane:PHB It allows a Wis save and is relient on your caster level. Bless is better.
Bless:PHB It relies on your caster level, but is still one of the better spells you can use at this level.
Detect Chaos/Law:PHB Not really useful, and someone else in the party could probably cast it a long, long time ago.
Divine Favor:PHB Like a personal Bless. One of your better choices at this level, despite relying on caster level for overall effectiveness.
Doom:PHB Causing a single target to become shaken after a failed Will save could have been nice on earlier levels on a class that can go SAD on Wisdom, but certainly not now and certainly not relying on you.
Endure Elements:PHB Could help on a trek through bad weather, but you are getting it late.
Magic Weapon:PHB If you or another martial companion in the party haven't gotten your magical weapon yet, this is how to fake it until you make it.
Protection from Chaos:PHB You are late to get it, but being able to say "Fuck you" to any possessing or mind control ability is worth every single spell slot when you need it. The AC and save bonuses against chaotic creatures aren't impressive, but a gladly taken cherry on top.
Resistance:PHB It was OK as a beginner Cleric orison, but as a 1st level spell on a latecomer as you this is a ridiculous price tag.
Shield of Faith:PHB Mid-term deflection bonus to AC dependent on caster level. You are not the best at using it, but given your crappy spell list this is one of your better ones.
Virtue:PHB It was OK as a beginner Cleric orison, but as a 1st level spell on a latecomer as you this is a ridiculous price tag.
Weapon Bless:OA A long-term one-shot Magic Weapon +5 for a single attack against a named single target, grandfathered in from OA without an official 3.5 update, and independent of your CL in its effectiveness. If you know who you are up against and don't suffer from time pressure, this is worth it
as a pre-adventure buff, as you can cast it in advance and then rest to prepare other spells.
*Divine Inspiration:BoED If it weren't so short and unpredictable in its duration, it could be worth the sacrifice. -1d2 Str.
*Twilight Luck:BoED It's a weak save bonus, but the fact that it lasts relatively long and comes with no strength sacrifice makes it the best of the sanctified ones at this spell level.
*Visions of Punishment:Champions of Valor Too short and your save DC is too bad. -1d2 Str
2ndAnimal Messenger:PHB An utility spell to prepare when you know you need it, and very likely something someone else in the party does better at less personal cost.
Bull's Strength:PHB If you want a short-term strength buff without disabling your spell-casting and skill use, this could be an useful Ki-Frenzy alternative for you.
Delay Poison:PHB By the time that you can delay poison, someone else can very likely cure it outright.
Eagle's Splendor:PHB Dragon 318 adds this to the Sohei spell list. Unless you are the cheerleader for the party marshal, a Cha boost ist an utter waste.
Honorable Weapon:DrM #318 Rokugan-specific adaption of Align Weapon that even there only has niche use to ignore certain DR.
Lesser Restoration:PHB Heals both ability damage and fatigue. Great to remove your own fatigue from Ki-Frenzy in a long fight, and generally a good just-in-case spell to have prepared when you don't know what else to prepare.
Owl's Wisdom:PHB Dragon 318 adds this to the Sohei spell list. This boosts the DC of your spells, so it isn't quite as useless as Eagle's Splendor, but if you use sohei spells that allow for saves you are in for a lot of disappointment.
Remove Paralysis:PHB When you expect someone to get paralyzed or slowed, this is good to have prepared. A wand or scrolls will probably do all other days.
Resist Energy:PHB Grants Energy Resistance 10 against one of five damage types you can pick as you cast the spell. Flexible and a potential life-saver.
Shield Others:PHB Allows you to take half an ally's damage on behalf of it. You could become the punching bag for the squishy wizard, but isn't that a bit demeaning even to you?
Warning:PGtF Bonus to Listen and Spot, Uncanny Dodge and not getting flat-footed for one single ally on the mid-term. Great on your dedicated scout, and a sweet buff on anyone else if flat-footed or dealing with invisible foes.
*Ayailla's Radiant Burst:BoED The blindness is very handy when it gets through, but in your hands it is more likely just weak AoE blasting. -1d2 Str
*Luminous Armor:BoED You will have a better armor bonus by this time, but the melee penalty against you combined with the long duration could make it handy even on top of armor. -1d2 Str
TFW you agree to make yourself the pin-cushion for the party squishies and actually cast Shield Others. 3rdCastigate:SpC A weak alignment based blasting AoE centered on you. The OA and the SpC versions differ somewhat, but neither version of the spell is really worth your time.
Discern Lies:PHB A poor man's Zone of Truth. Has some utility for interrogation, but when you can detain them, you can take a nap to prepare it as needed.
Dispel Magic:PHB Handy, but your weak caster level doesn't make you a viable threat to dedicated spellcasters.
Greater Magic Weapon:PHB One of the staples of the CoDzilla for a good reason. If CoDzilla won't cast it for you, and if you have seriously taken Sohei that far, one of your best spells at this level.
Magic Circle against Chaos:PHB AoE Protection vs Chaos, and good for the same reason the original was. As you aren't a summoner, you probably won't find use for the version that contains an extraplanar being.
Mental Strength:OA A short-term buff to Will saves for a single target. Didn't Protection from Chaos do the same thing better two spell levels ago?
Prayer:PHB Bless and Bane rolled into one, without the save against Bane. A bit short on duration for a standard action, but not the worst spell you can prepare.
Protection from Elements:PHB Similar to Resist Energy, but with a Stoneskin-like protection pool that lasts until it is all full, rather than to take a chunk of damage off for every hit during the full duration.
*Brilliant Emanation:BoED AoE blindness would be nice if you had a better save DC. -1d3 Str
*Celestial Aspect:BoED 1min/CL buff with a nice selection of possible effects, including flight, a holy longsword and a natural gore attack that can dismiss summons. -1d3 Str.
*Create Lantern Archon:Champions of Valor You can only get a single non-hazardous task up to 1 hour out of it without payment, but there is a lot of out of combat utility in a lantern archon as a translator, messenger and mobile Magic Circle against Evil. -1d2 Con
*Hammer of Righteousness:BoED Single-target force blasting could be helpful if you have no other way to deal with incorporeal creatures, but hopefully you have better options by now. -1d3 Str
*Path of the Exalted:BoED It's the "OK DM, where do we need to go next?" spell. You will have to abstain from divination spells 24h prior, but as this is a downtime spell this shouldn't pose an issue.
*Phieran's Resolve:BoED Very niche save bonus. -1d3 Str.
*Telepathy Tap:BoED It has some niche utility when you expect to need to eavesdrop on telepathic messages. If only the duration were better. -1d3 Str
4thDeath Ward:PHB Late immunity to death and negative energy effects is still a good immunity to have.
Discern Shapechanger:SpC Very, very situational, and the wizard got his as a 2nd level spell.
Dispel Chaos:PHB Good when it gets through, but your caster level and Save DC are much too low. The secondary effect of extra deflection AC would have been nice many levels earlier.
Divine Power:PHB A staple spell of CoDzilla for good reason. You are late to the party and lack CoDzilla's options to persist it cheaply, but for you it is one of your best options.
Fatigue:OA It costs a standard action. It allows a save. Fatigue and even exhaustion are trivial to deal with at at the time you get it, if the target isn't even outright immune. Next.
Freedom of Movement:PHB Being grappled and entangled are things of the past now. One of the best spells in the game, even if you get it late.
Neutralize Poison:PHB Someone else in the party does it probably better and for cheaper.
Order's Wrath:PHB Alignment-dependent AoE that doesn't enough damage even under ideal circumstances.
Remove Fatigue:BoED Out-of-combat spell that gives "the benefit of a good night's sleep". It explicitely heals fatigue and exhaustion, but says nothing about HP and ability damage. An oddity is that it very specifically rules out spell recovery for "arcane spellcasters". You could make a RAW argument that Psi Points and daily SLAs should still be recovered by this, as they aren't arcane spells. Please send me a card when you can talk any sane DM into it.
Restoration:PHB Lesser Restorations big brother that is good to have around just in case.
Spell Immunity:PHB Even if you didn't have to specify the spells to protect against, protection against 4th level spells isn't really relevant anymore at the time you get it.
Sustain:BoED Even if you still had an use for going without food and water by now, access to the material component would make it unnecessary.
*Animate with the Spirit:Champions of Valor Some weird good-aligned undead-but-not-really-undead creation. Might have some use if you have a suitable corpse lying around, but the 6HD cap makes the creature not even sword fodder if you got this from Sohei levels. -1d3 Str.
*Celestial Fortress:Champions of Valor Leomund's Secure Shelter with free Magic Circle vs Evil and Consecrate thrown in to make it more "sanctified". Getting out of a rest with attribute loss is not the perfect start in the day, but sometimes you are the last man standing to create an emergency shelter, so it doesn't hurt to have one of them prepared just in case. -1d2 Con.
*Diamond Spray:BoED AoE blasts that allow saves aren't your strong suit, and the material component is quite pricey for what it does.
*Holy Fire Shield:Champions of Valor Fire Shield, but half of the damage is untyped divine energy. -1d3 Str
*Luminous Armor, Greater:BoED More armor bonus, and sadly no increase to the melee penalty. -1d3 Str
*Sunmantle:BoED A weird mix of Daylight, damage reduction and light retributive damage. Not impressive at the time you get it. -1d4 Str