Do license tests and as many special events as you can first. You'll win a lot of prize cars and experience from these which you can use right away in A-Spec/B-Spec. This will save you from having to use a 100hp car for the first 15 races. I remember I did this at the start and I had enough money/exp to buy a Ferrari 458 - and I hadn't even finished all the Beginner races yet.
Also, buy the cars you LIKE not just the ones you think will help you win. I could have bought a better Ferrari but the 458 is one of my favourites and it did the job just as well as a Scuderia or Enzo would have.