I haven't read the whole thread but I just want to shime in to say that the E9x M3 is a complete beast on the track even with minimal mods.
Mine came with lowering springs. To which I added a pair of camber plates then got it aligned properly.
Also installed brake lines and high temp fluid. I swap race pads (in the front only) and run sticky street rubber.
The car is a total animal.
So those would be my recommendations for a starter/budget track setup that would make even advanced drivers happy.
The E9x is one hell of a chassis.