I think the Hommell you win in the World Compact Challenge is a sneaky good early car (the MX-5 can compete in that cup). I used it for several series, as well as the beginner Rally series. Even with 2 drive wheels, I dominated that circuit.
I would rather use 50k upgrading that than buying a new car. I did eventually buy an r34 Skyline Nur that'll upgrade to well over 700hp for about 50k a little later on though.
Also, sell the Toyota FT-86 Concept ’09 that you win in the FR challenge if you need some $ (if you have something that can win that, I don't think the miata can), it'll sell for 90k