Just to be clear, I would love, absolutely love to race karts..on custom tracks, just not on GT5 tracks other than maybe the Autumn Rings.
My 5 ideas would be only 1 idea:
1. 380-420 PP CS/SH with a single make road car, FR or MR, Premium or good quality Standards like the Mugen or Spoon S2000's.
I haven't found an FF or a 4wd yet, in it's spec form, that doesn't understeer like a bulldozer after the update. I just tuned a 370PP Civic for the Parity League, threw 200 kgs of ballast on the back and it still understeered. Before the update it would have been uncontrollable with the settings I used. If you can find one that doesn't then they are in the mix, if not, stick to FR's and MR's.
Vintage cars are fine, but in spec mode many of them have terrible gearing and you run around some tracks only using 2 gears and way off the powerband half the time. I've driven dozens of them extensively in the VCRC and in the Parity League. There are some exceptions like the Nissan 240RS and the good sports/super cars but not many. You can take 10 of them as was suggested, all at the same PP and they could be 2-3 seconds apart. If you pick one car that's fine, but trying to homologate more than 1 is a difficult at best...10 is pretty near impossible.
The best option by far for close, clean racing, is lower pp premium cars. Cornering speeds are lower meaning less chance of error and more time to react. CS ups the skill quotient a bit, SH makes the cars a little more tame and everyone more consistent.
There are many good cars that fit this bill, you only need drive them at the specs you want to use on the tracks you're thinking about using.