If you have trouble finding the right car for an event look at the "typical entrants" next to the tyre restrictions when about the enter the event. They are arranged with the most powerful at the top and you can usually tell which is the best by the one with the most power BUT also not as much weight. For the Truck race for instance (where you need to buy a used truck - theres no new ones) The Toyota Tacoma X runner is the best because it is much lighter... I bought the Chevy SST instead because it was there... it's probably the second best for the event, but very expensive, so not the best choice money wise.
Failing this, do the event if you have a car that can enter it and see which car dominates. Sometimes the best cars won't be included so you may have to do it more than once. I usually only do this if I enter and fail badly because my car sucks, that's only happened once though, the Vits race... I ended up sticking with the model I had because it was lighter and tuned it a bit more. For my crappy b-spec driver I chucked a big turbo on it
Another thing, don't sell prize cars unless you know you don't need it. Most are useful for later events... like the Dome concept car for the Classic Japanese race for example, with a bit of tuning it dominates. You also need to keep these cars after winning for your b-spec career, although with the bad b-spec drivers you may need to tune more and buy worse tyres because of tyre restrictions in b-spec.