One that attracted as many hot girls as it would car dudes.
Seriously, i'd have to go with a custom lightweight tube chassis, carbon fiber body panels, full lexan windows, air conditioning that would rival a meat locker, all aluminum twin turbo v8 matched to a fully sequential 6 speed transmission and getting 60 mpg AROUND TOWN. I'd also have to incorporate a crazy wide body kit (while keeping the stock lines as much as possible) and underbody redesign for higher terminal speeds.
Did i mention free, and with a totally hot chick that constantly told me how hot me and my awesome Z make her. She would also have to pump my gas.
-Rustrocket