For your price range, you're going to be doing a lead-acid conversion. The price of a lithium pack & battery management system
alone would probably put you over $10k.
I trust you've had a look through the
EV Album?
I voted for the pickup, because it keeps the pack out of the passenger compartment, and has more options for aero mods. Plus as Ryland points out they've been done by many people, so you wouldn't have to reinvent the wheel. I'm sure there are even kits available.