As far as cheap, fun, reliable sports cars, it's hard to beat the Miata.I had a mint 99 that I put an Eaton MP62 blower on. Made around 190rwhp, and made the car into what it should have been from the factory.I just sold it a couple weeks before Christmas. I just have too many cars and wanted the space.
Stay away from those Porsches unless you want to deal with IMS bearings.The Audis will likely be maintenance whores as well unless you get one with LOW miles. The Merc would be okay, as would the Nissan. BMW are maintenance whores also, but cheaper and easier generally.