I wouldn't buy another X10. You already have the proto, that should be good enough. I wouldn't spend all your money on just one or two cars either. Buy an assortment. Like maybe a good race car, maybe a muscle car or two. Buy whatever will help you progress through the game. My personal favorites are:
Bentley Speed 8
Suzuki GSX-R (I think thats what its called)
Enzo (It's an aquired taste, lol)
TVR Griffith
Shelby Series One
Chevy Camaro '69 (RM it)
Chevy Camaro '10
Nissan R390 Road Car
Nissan Yellowhat GT-R '08 (or any Super GT car)
Just some of my daily drivers and good cars imo. Hope this helps you out.