For a 15K budget, I'd have no problem building a car from scratch. If it's a cheap chassis you're after, then I would check out
Everything you need to build your dream street rod A t-bucket? Yep, they are light, cheap, and you can build your own body however you want to. My 82 Chevette makes 35 hp to the wheels and weighs 2400 lb with me in it. I figure something that weighs 1200 lb and has a 15-20 hp electric motor should do about the same thing, and be fast enough for freeway use. Once upon a time, I was building a T for drag racing, and I can tell you that the chassis, suspension, steering, and front brakes weigh no more than about 300 lb. If it were me, I would build a three-wheel car, that way it would be classified as a motorcycle which means you could get by with fewer regs from the motor vehicles dept. As for a body, I'd design it as close as I could to 1/2 an airplane wing and call it good. Good luck!!