Wow, those restrictions are tough. I'll try to help out... (God I hope you didn't have to pay the LA on Drow, or can at least
buy it off later.)
First off, if you have access to Complete Mage, it has the best of the (very few) printed Swashbuckler alternate class features on p. 32, Arcane Stunt. You need a rank in Knowledge Arcana (which costs 2 skill points as it's cross class), but you can swap Grace for the ability to use spell-like abilities. First you pick one, then when Grace would raise to +2 and +3 (at levels 11 and 20) you get to pick additional spells from the list. The casting time is a swift action (except for feather fall, which is an immediate action) and the duration reduced to 1 round. The duration icreases by a round each at levels 11 and 20 as well. The spell-like abilities you can pick up are: blur, expeditious retreat, feather fall, jump, or spider climb. You can use any of the SLAs you choose in any combination up to 1+int mod times/day. Blur, I think, would be the most generally useful buff to grab, though all are handy.
For your one prestige class, if you can hold out in Swashbuckler that long, I think Dervish (Complete Warrior) is your best bet. Go with two weapon fighting and buy up ranks in perform (dance). It's worth it later on. Really, the only no-brainer class skill Swash even gives you is Tumble anyway.
If Tome of Battle is an option, you can get maneuvers from there as feats or in item form that may be useful. Let me know, and I'll suggest some.
EDIT: Two things on items. An amulet of natural armor +1 is 2000 gp, someone said it was 1000 gp. There's no reason to ever get one before a ring of protection +1, which costs the same and adds to touch AC.
Also, if your DM is that harsh on gold, that really hurts non-casters. Will he allow Vow of Poverty from Book of Exalted Deeds?