Min/Max Boards

It's not a buff but Divine Defiance (FC2) lets you counterspell as an immediate action. You can also take Church Inquisitor (CD) at level 4 to get the Inquisition domain to boost you dispel checks.
Another option is Divine Metamagic (CD) for Quicken Spell.

Mind Thrust is [Mind-Affecting], which you want to get immunity to anyway. Mind Blank is the usual solution, but it's usually a high level one. There's several types that are naturally immune (plants, undead, constructs) and a whole bunch of spells, items and class features that grant full or partial immunity.

For more immediate protection get your armor enchanted with Mindarmor (MIC, 3000gp) for a +5 bonus to will saves as an immediate action. Combine this with an armor Crystal of Mind Cloaking (MIC, 500-10000gp) for another +1-5 bonus against mind-affecting and a reroll 1/day on the greater crystall.
Both of those are pretty good investments for warrior/rogue types anyway.

Null Psionics Field has the drawback that it disables all your magic and psionic items (if you're playing with transparency, which is the default).

Why not use the Darkwood Buckler as written (a light darkwood shield)? The only difference compared to a buckler is that you can't wield a weapon in your shield hand, which primary casters don't care about anyway.

or custom-make a sword that just emits silence from itself (the user will be silenced too but a rogue won't mind)

The cost would be?

Namarra (MoF, 30315gp). +2 keen longsword, casts silence on itself at will. Substract 18000gp for the +2 keen, 315gp for a masterwork longsword and the silence effect ends up costing 12000gp.

Fullblade (A&E) or Minotaur Greathammer
Spiked Chain
Strongarm Bracers (MIC)

By the RAW it works. You can cast a 2nd level spell, you qualify. The FAQ says it doesn't, but the FAQ isn't official rules, it's opinion.
A lot of DMs don't allow it. Most people refer to it as "Early Entry Cheese" so yes, it is cheesy in most peoples opinion.
On the other hand it makes dual progression classes like Mystic Theurge and Arcane Hierophant a lot more viable, because they suck otherwise.

Those are the facts. Make of them what you will. In the end it comes down to "ask your DM". It depends on your campaigns power level if it's OP or not.

Yes... which is why i'm looking for options to survive high level play without immunity to mind-affecting.   
My party also enjoys my Inspire Greatness and the Heartfire Fanners free feat and free metamagic songs. Those are mind-affecting too.

Also, items with permanent Mind Blank are expensive. If there are ways to get the necessary protection cheaper i'll certainly not throw my gold away. 

I'm aware or PfE. Extended and with a rod of lesser Chain spell it takes care of possession and dominate for the whole party, with a few 2nd level Pearls of Power just in case. It requires a bit of forethought but we've done alright with it so far, and the summon protection is pretty handy.
A cheap and effective solution imo since the whole party can share the cost to get it up asap. Being dominated sucks. 

The problem with it is that it doesn't protect from Illusion(Pattern) and (Phantasm) spells, Compulsions that don't grant control like Irresistible Dance or Touch of Idiocy, other Enchantment spells like Mindrape, a whole host of psionic effects and fear effects. And our DM will take advantage of any vulnerability, so that's quite a bit.
So far we've been using the Mindarmor enhancement from MIC and Greater Armor Crystals of Mind Cloaking (and save buffs in general). We're still alive, but it just isn't the same as Immunity.

The only one of those that i have a way to adress is fear. Even then it gets complicated fast if you have to get a seperate protection item/spell for every little thing, so i was hoping for a simpler solution.

We're currently level 11-12 and have a party of bard/crusader/heartfire fanner, wizard/swiftblade gish, summoner druid and an artificer/cannith wand adept so spell & item access shouldn't be a problem. 
It's also not quite an immediate problem, just something that came up after the last session because we're getting near the point where Mind Blank is a thing.

It's widely accepted that any high level character should get Mind Blank in one form or another.
Since all bardic music is mind-affecting that pretty much means your bard becomes a crippled sorcerer unless your entire party neglects a crucial defense.

What i'm looking for is duplicating the effects of Mind Blank as much as possible while still allowing bardic music to be effective.
What i have so far:
Ring of Mind Shielding: protects from Detect Thoughts, Discern Lies and alignment detection - cheap enough for pretty much everyone to get but pretty limited

Exalted armor enhancement or persisted Protection from X: protects from mind control/possession - Exalted is a +3 enhancement and only works for good characters, PfX can be dispelled/run out and also costs significant daily resources even with DMM or Incantatrix.

Nondetection: protects from divinations - has a material component (though it's not too much of a problem at high level), offers a CL check so it's not absolute

What i'm looking for:
Protection from Divination, preferably absolute (no CL checks or saves), in item form or long duration
Protection from as many negative mind-affecting effects as possible, with the same conditions, while still allowing bardic music effects.

The Robe of Mysterious Conjuration (MIC, 10000gp) lets you use Summon Monster 3/day as a standard action by sacrificing a spell. Not a main weapon but a handy and relatively affordable backup.

Heighten does work with reserve feats as long as you have an open spell slot but the beguiler list is pretty limited when it comes to spells to fuel the damage dealing ones.

Prestige Bard should add at least the core spells that are unique to the bard list, the rest is up to the DM.

Unique Spells

The bard, paladin, and ranger spell lists contain a number of spells that don't appear on other classes' spell lists. In general, any character who enters one of these prestige classes should gain access to spells unique to that class's spell list, at the same levels indicated for the standard class. At the game master's discretion, spells unique to that class's spell list found in other books may also be available, but on a case by case basis. The game master may require such spells to be researched or learned specifically by the character, rather than simply making them freely available.

You can get some mileage out of Inspire Courage with spells and items (about +4-5 off the top of my head).Dragonfire Inspiration may be an option if you can spare the feats to get a non-fire bloodline.
If your DM is amicable to adding other sources on a case by case basis you could also look into Heartfire Fanner (Dr314) and/or War Weaver (HoB) for a strong support build.

Rainbow Servant is another option though it comes online rather late (ECL 15). You can theoretically get in at ECL 2 with early entry cheese but i doubt your DM is up for that.