Imho PS7, CS1 or CS2 are a good choice. (haven't worked on CS3 for now)
I bought CS1 a few (2?) years ago, it still has every feature I need![]()
On work I'm on CS2, the differences aren't that big (at least it wasn't worth the money for an upgrade)
The only interresting part on CS3 seems to be the support of 3D files, so you're able painting directly on your model(but to be honest, zbrush can do the same...) but this is just interresting if you're in game development or moding, otherwise older PS versions will do the job for sure.