This is a tough one. Fun, reliable (or at least fairly simple to repair), four-seat convertible for $15,000?
The closest fit is probably a Ford Mustang, though I suspect it's no more your sort of car than it is mine.
Next closest would be rolling the dice on something European (3-Series, A4, CLK, 9-3, C70, Eos).
Drop the convertible requirement, and the four-seaters closest in character to your favorites are the Mazda RX-8 and Scion FR-S (which would likely be a bit over your budget, but some asking prices are close).
The Scion is noisy, not very comfortable on the highway, and has a very tight back seat.
The Mazda is both more comfortable, more practical (roomier and easier to access rear seat), and more agile, but has poor fuel economy and just so-so reliability (including some engine failures). I'll soon own one myself anyway. Quite a few owners seem to do their own repairs, as rotary engines really aren't all that complicated.
The fewer miles you drive a year, the more sense the RX-8 would make.
If RWD really isn't suitable for your needs (in Colorado), perhaps a Subaru WRX, MazdaSpeed3, or Civic Si.