If you can manage it, get a '91 325i 2dr with the sport package (they didn't offer the iS in '91, go figure). You'll have a LSD to your advantage right off the bat, as well as a pretty competative car. A close second would be the slightly older 325iS, as mentioned.
The class that you would be in is D Stock (DS). The other cars you would be racing against:
http://www.moutons.org/sccasolo/Lists/stockc.html
This class is pretty competative on the national level and has a decent variety of cars in class that would be competative. Nationally, though, the new 330Ci and the Integra Type R rule the class. Locally, you'll mostly find WRX's in class. A well-tuned E30 325i would certainly give the WRX's a run for their money. I'll be running DS in Pittsburgh with my E36 next year.
Just be careful about anything that you might do to the prospective car, as changes that you make will put you in a different class. I would recommend racing the car in stock class if you wish to be competative. Also, it saves money, and is just as fun, I'm sure.
A thread you may be interested in, although it pertains to e36:
http://www.sccaforums.com/ubb/ultimatebb.php/topic/18/851