I have a fully tuned coquette, engine mods, turbo, brake mods... everything. I decided too late i would prefer a carbonizzare, spent too much money on the coquette already.
In my experience it doesn't matter that much anyway. If there is a race where personal cars are allowed and catch-up is disabled, if you enter with a tuned car (engine mods, turbo), once you get away, most people will never catch up anymore.
Also mentioned by Coletus a few posts above, get a tuned bike... you will win almost all the races.
Regarding selling cars with a value higher than 50k... i have the same issue... i think i'll drop the ones i want to get rid of into the ocean...