Cams are just not cheap for an e36 M3, sorry. But take a look here, www.turnermotorspot.com
As far as body differences, the 4 door version tail lights (like Rooz said) are not as lengthy and look a bit mroe like a square compared to the rectangular types for the coupe.
The front turn signals are different. I cant really recall the difference but the 4 door and 2 door front corners are not interchangable.
Also another difference between the coupe and sedan is that the coupe, has 2 doors, and the sedan, has 4 doors. Go figure!
And as far as boost goes, i'd stay away from it for awhile. The engines just arent meant for high boost at all. I think your car's compression ratio is like 12.1:1 or somethign ridiculously high. Try and stick to bolt ons and other addons for the time being. Reliability will take a downhill spin also with boost, and since you are sucked dry for money I think it's smart to stay away for a bit.