I've just moved back from Florida mate. Convertibles are a no go in that heat, not only do you never actually use them because you want the AC on full whack, but they fade really really fast. I was drinking a Mustang GT, great car, decent gas mileage, sat around 20mpg and that's intermittent hooning, also, it's based on a US Gal so it's actually more than that if you convert to a IMP Gal.
If you're out there for a long time, buy new, if not, buy used. The depreciation of new cars is worse out there than over here, and you can pick up a bargain if you get a couple of years old.
If you really really want to have fun though, and you have a decent wedge, Corvette C7. You can get a bottom line for about $60K, and if you do come back to the UK, as long as you've been gone for 12 months and owned the car for 6... No import tax.