I can't think of a $500 dollar car that meets all those requirements, but the closest would be an old GEO Metro.
Personally, I'd up the budget 3x and shop for a well maintained CRX HF. They are gutless, all high mileage by this point, and Honda has earned a dubious reputation as a long-term parts supplier...but they are cheap, plentiful, and full of eco-modding possibilities. You could easily get 50+mpg, you're car history is spattered with Hondas, and there is potential for 60-80+mpg with a D15z1 engine swap.
Unfortunately, the recent gasoline price spike has dried up a once large market...at least around here...
- LostCause