Stock:
SS-Porsche GT3 & C5 Corvette Z06. It remains to be seen which will prevail.
AS-C4 'Vette, Boxster-S, S-2000, E46 M3 CSL (and maybe an Evo or Sti on a slick and/or wet surface).
BS-RX-8, 944 S2, 968, Mr2T, E36M3.
CS-'99 Miata Sport Pack., Mr2S w/LSD
DS- ITR, 330, IS300
ES- Mr2 n/a, '94 Miata w/right options (ask Jesse for specifics)
FS- Mach1, Bullit (the new GT may have potential)
GS- '05 Mini-S, current Celica GT, '05 RSX-S
HS- Mini, Audi 90
Competitive where is the biggest question? If you just want to win your class locally than H-Stock is wide open - buy a Mini and try to butch it up a bit, huh..
I've read through this thread and just want to say that the Z is a poor choice as an autocrosser. Yes, you can mod it and end up in B Street Prepared where it'll still get smoked by the C4 'Vette (and probably the S2000 too). The Z isn't that competitive and once you graduate to race rubber it'll requite a hitch and trailer. It eats up front tires due to the lack of available front negative camber and the insurance rates are high.
SS is out, based on price.
In AS the S2000 and '04 (back) Boxster-S will only be competitive with a top level driver. The C4 will dominate at most venues but it's too old for you.
Then there's BS. Under your origional parameters I'd say you'd be o.k. in a RX-8. If you can stand the rotary racket and the hurrendous gas mileage they'll be a competitive B-Stock car and once your driving is up to snuff there are many aftermarket parts available to make it faster under stock rules (double adjustable dampers, adjustable front sway bars, and titanium exhausts). You can put a set of wheels in the back seat and still have an entire trunk left over (+carry a passenger). You can find one in the low 20's and still finance it for 60-72 mos. Insurance isn't too bad since it's a 2+2 and a 1.3L.
Skip CS if you're not into convertibles or Miata's.
DS, well ITR's (especially unmodded and abused) are few & far between. The IS300 & 300 would be great daily drivers and still are plenty fast on the autocross course...
FS, get a new Mustang GT for 24ish and be one of the only guys around with one. I think it'll be a good FS car but I hear there may be a 1/2" wider wheel on it for '06.
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T.Morrow