In order of cost to go racing: ITR,S,A,B,C. ITC being the "cheapest"
For most people, it boils down to what car you'd love to build, what car lands in your lap, OR what car comes out on top once you've spent months comparing brake type, engine, weight, gear ratios, suspension type, aftermarket support etc.
I'd suggest buying a built car over building one. Buy it, be out on the track the next weekend. Flog the heck out of it learning to drive and you'll figure out what car is "the" car to have in the process. THEN build the car you want and when it's done, sell (what's left) of the first one.