I don't know what the best autocross setup is but if you keep your drop around 2" it shouldn't mess up the factory suspension geometry up too much. Any lower than that and things can get expensive in a hurry to straighten things out again. S13's seem to really love front camber so 2-3 degrees to start out with would probably be good and crank in 6-7 deg. caster. Caster is free camber without the tire wear and loss of straight line braking. Rear tow could be set 1/16 in for some added grip and front should be 0 to 1/16 out for good turn in response.
Also you may want to check into some Isis control arms to go with your coilovers. They are, in my opinion a really good bang for the buck. I've run a set on my daily now for a year and they have preformed really well. Also I think the Fortunes are pretty good too. We're running a set on one of our project cars right now and they show promise, although it's hard to get a car to set high enough especially in the rear with the stock lower hats. And replace the rear cradle bushings with Energy Suspension S14 ones. You have to remove just the rubber middle section and maintain the outer steel sleeve to use them but once you get them in there it's really worth all the effort. Replacing the remaining bushings and ball joints, wheel bearings, and steering rack bushings would be good too. And don't forget about overhauling the brakes too. Braided lines really help out there along with a master cylinder stopper. Those two really crisp up the pedal. I'm a big fan of maintance if you can't tell. These cars are 20+ years old now so they need some love if you plan on driving it like you stole it.
These guys however are experts. All they do is set up cars for road course use and they have experience with the S13 chassis. Give them a call. I'm sure they can steer you in the right direction
http://texastrackworks.com/
Hope this helps