What are you racing now? CSR & FA are the real deep end of the pool, and not a good place for a novice.
CSR has the transition to P1 & P2 (formerly SR1 & SR2) hanging over it's head. This has not been wildly popular, especially on the faster P1/SR1 side which is likely to hurt the class. But the shakeup will likely create opportunities for the clever, and has put a fair number of cars on the market at decent prices. If you like rules stability, the clear choice is FA.
There are no kits for FA or CSR (see the deep end of the pool comment above). If this is what you are looking for you should be looking at FB / F1000 conversions, F500 or 600, and possibly P2/SR2 conversions/builds. Home builts and kits have long since been overtaken in the top Formula and Sport Racing classes by professionally designed & built cars.