A 90 318 shell would be legal, no problem, but you will have to swap the drivetrain to make it a Pro3. You have to run the M20B25 motor (2.5L I motor), which I think was in pretty much all 325's from '87 to '91. Fuel cell is not mandatory. Most of us have run the stock tank to begin with, and some have chosen to stay with it. No real advantage 2 vs 4 door cars, although Mike and Lance are going to argue that 4 doors are cool.
I believe there was that crazy "Super ETA" in 1988 that could throw a curve ball at you when shopping for engines. Easiest way to tell when first looking at a donor car is the tach: redline for a 2.7 eta motor is at 4500; the correct motor, M20B25, shows a redline at 6000.
I'm no expert on the minutia details between the 318 and 325, but my understanding is that you would be better off to find a true 325i or 325is and start from there. At a minimum, you would need to change out the entire drive train including the differential and engine ECU. I don't believe any Pro3 car in the fleet started as a 318...
True, there seems to be a few Pro3 cars for sale, each with its own reasons for selling. For as many that are currently for sale, there are just as many under construction...that's to be expected when our 'fleet' now numbers greater than 40!
Michael Lord is selling his rental Pro3 - built by TC Motosports and a very capable car.
Kevin's car is fast (so is Kevin!) as he sure kicked my ass in Mission this year!!
4 door vs 2 door? Well, all the cool kids have 4!
I swear the 4-door is heavier...or rather, it's more difficult to get it down to minimum weight. Of course, if you ask my Crew Chief, he's quick to point out that if he drove the car, we would be under weight! Maybe if I laid off the biscuits and gravy a little bit more...
Hit the sites listed by Tom or feel free to shoot any of the Pro3 guys an email.