Check the prize car thread, it'll help you plan your purchases and save you probably literally millions, especially if you love Zondas...
I think the best starting car I've found was the Civic Type R, ok so it wipes out all your starting cash but so would a used car after you recondition the engine and straighten the chassis. It'll do the Sunday Cup, FF Challenge, World Compact Car Cup, Clubman Cup, Japanese 90's Challenge and if you modify it enough, maybe even the Japanese Championship, Turbo Challenge and the Polyphony Digital Cup, though I have absolutely no evidence to support this other than someone else saying he managed to almost beat a load of supercars with a Civic.
I'm going to try it some day, just not now.
I'd really recommend B-Spec mode, though, it's not easy or cheap but some of the prize cars will help immensely and it'll be worth it. Highlights include: Mazda Furai, Toyota 7 Race Car, Pagani Zonda R and such, the prizes are far better than the A-Spec ones.