Weren't you talking about doing a 6 speed swap? If you're just driving 5 miles to work each way, why are you concerned about gas mileage-- if that's all the driving you do in a work week- 50 miles? So that's about 1/8th a tank, 1/4 max. Plus, you won't really get that much better gas mileage if you move to a 525 or 535.
I can't help you on the amount you'll be able to sell it for, although I'm curious what a reasonable price is for it. My guess would be between 5-6k.