Apostle of peace has... issues, including a variety of balance and rules issues. I don't know how much your DM is changing the rules for you, so I'm just going to go down the list of basic problems and you can tell me what's happening with them.
1) The Vow feats and gray guard. Since you lose the feats if you don't follow them and until you hit level 10 gray guard isn't going to help you, because the feats don't give you the chance to atone if you voluntarily break them, you are going to be every bit as crippled, and harmful to your party, as any other apostle of peace.
2) Even once you hit gray guard 10, Vow of Nonviolence will still screw over your party. "Your purity is so great that any ally of yours who slays a helpless or defenseless foe within 120 feet of you feels great remorse. Your ally takes a -1 morale penalty on his attack rolls for 1 hour per your character level. For each helpless foe slain, the attack penalty increases by 1, to a maximum equal to your character level." (BE) RAW, sacrament of the true faith affects you, not your allies, so they'll still be taking massive morale penalties for killing.
3) Apostle of peace doesn't lift any of the Vow restrictions, RAW. It says, "As part of their sacred vows, apostles of peace forswear the use of armor, though they may wear magic items that protect them (such as a ring of protection or bracers of armor)." Meanwhile, Vow of Poverty prohibits the use of magic items. As it is, this is a class-based conduct restriction that would cause you to lose your apostle of peace features directly if you violate it, but violating Vow of Poverty's conduct requirements causes you to lose the feat which in turn costs you your apostle of peace class features too.
My next question is: what do you envision this character doing? The "normal" way of using Vow of Nonviolence is by finding spells that don't technically kill their targets like baleful polymorph and using the +4 to save DCs to incapacitate enemies without killing them. At that point you can get into fun arguments with your DM about what counts as killing, "great harm," and so on. Spells that put enemies in stasis like imprisonment? What about feeblemind, which doesn't technically do ability damage? And so on. Do you care about mattering in combat at all? The apostle of peace spell list has adequate though not strong out-of-combat utility but only a very few spells that can be used in combat other than defensively, including buffs that help your allies incapacitate or kill things. With bonuses to Bluff, Disguise, Intimidate, and Diplomacy from gray guard and the Vow feats, and the prerequisites of gray guard and apostle of peace, paladin levels and Diplomacy 6, Sense Motive 4, Knowledge (religion) 8, and Concentration 10, you're pretty much forced to a social-skills focus, probably Bluff, Diplomacy, and Sense Motive since I'll doubt you have the points for anything more than those, the prereqs, and keeping Concentration maxed.
Since I don't have a good sense of what you want your character to do, in or out of combat, my other advice is somewhat limited. With flaws, you should have no trouble getting the required feats in time, so you should be able to enter apostle of peace at 8th level. As you can enter gray guard at 6th, you need to get to gray guard 10 ASAP, and it doesn't advance spellcasting until 2nd level, you want to take one level of gray guard ASAP. You need two levels of paladin for gray guard, but more levels doesn't give you much of anything you care about. That leaves you with four levels of classes to get a +5 Will save and the prerequisite skills. Obvious candidate classes include cleric, preferably cloistered, and monk, since you can't wear armor anyways, but there are also non-core options like marshal and, oddly, swordsage. Your first level should probably some class that gets more than 2 + Int skills. Spellcasting classes will increase your caster level with apostle of peace, and if you take all four of those levels in spellcasting classes, with Practiced Spellcaster you'll only be one level behind in casting.
1: ?; Sacred Vow, Vow of Peace, Vow of Nonviolence
2: ?;
3: paladin; Vow of Poverty
4: paladin
5: ?
6: gray guard; ?
7: ?
8: apostle of peace
9: gray guard; Practiced Spellcaster
