I can help a little...
S2 is a product from Primo along with a bunch of other stuff.
You don't need "Clean Theme" but they say it works flawlessly with s2 and of course they can more easily assist with issues if you do use Clean Theme so if you need a theme it would seem a reasonable choice.
That said, I am not using Clean Theme and have had no issues. Also, all of the issues I have seen reported in the forum are plugin related. I really don't think I have seen a theme related conflict but I am sure they exist..
As far as buying, this is the place :
http://www.s2member.com/prices/And if you look around on the net you may find a coupon code which can save you a bit.
DP