I know the Miata has a reputation as a chick's car but it is the best of the bunch that you've listed, taking into account being able to find one.
S2000, good luck finding one that hasn't been thrashed, and I still don't think it's as good as a Miata and certainly won't be as cheap for a good one.
Forget the Porsche. Google "Porsche IMS" and you'll see why. I know the Porsche addicts will all start screaming "it didn't affect all years" and "not all of them did it" but the horror stories, and the failure of the manufacturer to provide any support, all add up to RUN AWAY.
The Mercedes is not a sporty car despite its appearance. Neither is the Audi TT (front wheel drive). Yes, the Audiphiles will scream "all wheel drive". No, it's front wheel drive with a little bit of help from the back. Transverse engine. It's a VW Golf - which is not a bad car, but I don't think it's really what you are looking for.
I don't know much about the BMWs.
And now, I have an off the wall suggestion.
Go to your local Chrysler dealer and test-drive a Fiat 124.
Yes, I know it is the same underneath as the new Miata. That is not a bad thing. Yes, I know the journalists rag on the 1.4 turbo engine. They're full of it. Yes, I know the dealers are having trouble selling them. Means you can probably argue with them about the price tag ... and you'll get a new car with a warranty and minimal headaches and probably 0% financing. For all those worrying about "Fix It Again Tony" ... remember, this car is a Mazda. The only thing Fiat is the engine and the badge, and the engine is good.
Go drive one.