if you shop wisely you can get a STR6 for a very attractive price. What you get is a hand-built totally blueprinted AMG engine with a Karmann/Mercedes sports car thrown in for almost free ! Seriously, the replacement cost of the long block is something like 24K !!!
As for the lack of an aftermarket; so what ? These cars are so highly tuned and finely balanced they are awfully hard to improve on without screwing up something. If you want something you can endlessly tinker with and bolt on parts, get a Mustang.
If you want a great looking sports car with supercar performance and a bargain price...by a STR6 !
Slim