I love photoshop, but I wouldn't spend that amount of money if it's mainly to do bevelled buttons and so on. There's a variation on The Gimp called
Gimpshop (still free), which is basically a branch of the program that's aimed at being more photoshop-like.
I wouldn't worry too much about the tutorial aspect, as the actual techniques used will be much the same across most programs and making buttons etc isn't a particularly technical exercise.. I'm sure you'll be able to figure out how to apply the techniques to your particular program.
Alternatively, there's a bunch of programs out there specifically for making web graphics. Most are quite limited, though.. wouldn't be much use for doing things like backgrounds or banners.
If you're set on photoshop, go for standard CS3 (or an older version if you can find it).. you're unlikely to need the extra features of the extended version. Be careful buying second-hand copies of applications - you never know if it's already been registered to someone else and if that person can transfer ownership. That can limit you if you need to be registered with the company in order to receive patches, upgrades etc.