So, after your criticisms, I have decided to modify the Swashbuckler.
A Swashbuckler who starts off with, let's say, Dex 16 (increased to 18 by leveling) and +1 Ghost Touch Mithral Breastplate which gives +6 AC for 20,350. AC 20 so far.
A Ring of Protection +2 (8,000) gives AC 22. 28,350 gp so far.
Gloves of Dexterity +2 (4,000) now give AC 23. 33,350 gp so far.
Ioun Stone for +1 AC. We can has AC 24. 38,980 gp spent
Combat Expertise is required to get into Dervish. Can boost AC up to 29.
Note: Defending weapons can boost this further.
AC vs Incoporeal is 23 base (Dex, armor, ring), but can be boosted up to 28 with Combat Expertise.
Touch AC is 18 base, but can be boosted up to 23 with Combat Expertise.
Flatfooted AC is 18
Your stats were:
Normal: 26
Incoporeal: 25
Touch: 21
Flatfooted: 13
Total GP spent on AC: 38,980
Total GP left after boosting AC: 49,020 gp. Which is still a bit more than the 42320 gp you had left over.
A pair of +1 Defending scimitars for about 16,630 (can boost AC by 2), +4 item to increase Str, and +2 item for Con, and Boots of Speed use up the remaining cash nicely.
So you have higher initiative by 3. That's pretty much the only area you clearly come out ahead in.
Meanwhile, the Swashbuckler/Dervish gets more Con, more Str, Boots of Speed, higher AC... before Dervish abilities are factored into it. Your Swashbuckler6/Devish4 is weaker than a straight Swashbuckler alone in pretty much all areas. This is the definition of a -2 tier class.