Oh didn't know it had to be a 4 seater convertible, that changes things
Hmm, I guess it depends what you're really looking for. An E93 M3 would be nice, but not sure if it's fast enough for you. They're the cheapest in the lineup, can be had with a good DCT and once bulletproofed, are very reliable. I think you'd be in under $35k USD all said and done, and you can do fun mods like exhaust (don't recommend test pipes on a convertible) and carbon plenum. I love the idea of being able the S65 in all its glory with the top down.
I think I'm on the same page as you though, I cannot justify the price of a 997 GTS either. It starts creeping up to some other seriously tasty used Porsches.
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Current:
16 F82 M4 GTS, Black Sapphire/Black, DCT
08 E92 M3, Sparkling Graphite/Bamboo Beige, 6MT
07 E85 Z4M Roadster, Alpine White/Red, 6MT
99 E36 M3, Techno Violet/Dove Grey, 6MT