For the most part a tuner car such as the S2000 is a horrible investment financially speaking. They will almost never return to their original value let alone appreciate. The rare exceptions would be a low mileage Toyota Supra or an early Nissan Skyline(R32 or before). You would have to baby them though, restricting your fun.
If you really want a sports car that will appreciate, buy a muscle car. The have a great appreciation history. I would recommend something pre-1980 or post-2005. With a late model one, such as the Challenger you could probably get double what you paid for it in 20 years as long as its in great condition with low mileage. With an older model as long as you buy slightly beat up and are able to restore it you could easily get double what you paid as soon as the restoration is complete.
