This is pretty much what I do for every GT. With GT5 I'm using the same car for A & B spec versions of a race, just in different states of tune, and using 1 per race series as there don't seem to be as many as before and can have closer racing. Takes me a bit longer (just completed all Beginner races) but I find that I pick up lots of cars along the way that I want and I get to savour the game for longer knowing that at the end I'll be able to go back and buy the rest of the cars I crave. I haven't been following it as rigidly as in previous GTs though and bought quite a few cars already.
I used the Autobianchi for the Euro classics, I think I won that in the FF race but it may have been the compact cup. I had to give it enough extra power to not get too outgunned on the Madrid straights in A-Spec. In B-Spec it was pretty much tuned to the max so my B-spec drivers could overtake on the straights and not have to rely on getting round the last bend on heat to get enough speed as they usually understeered into the barrier in their excitement every lap.
For the Japanese classic I guess could have used an Isuzu 117 (unless this is the series I won it in), or tried to get a Honda Z fast enough (unlikely) but I chose to buy a 1970 Skyline as it looked good and fitted what I wanted.
The Classic car I used the Isuzu 117 Coupe which mainly needed a bit of power and gearing to reach 150 down the Mulsanne to do La Sarthe in BSpec and enough weight & chassis work to get Eiger & Tsukuba. IIRC though the Isuzu would have come from the Japanese Classics
It should be possible to go a long way on just prize cars, especially if you bought a Lupo with your first car like me, and got the Chrome Line Murcielago in the the CE like me.

Although I am not doing it properly this time, from GT4 I found the easiest way at the start was to go through the cars in the order I won them and run any races they qualified for, rather than doing the races in order and scouring the garage for a suitable car. This way you get a large bank of prize cars. Also golding licences and specials will get you more cars
Good luck