Can I use iPhone 4 without it being activated?

My friend wants to sell me his iPhone 4S for 50$ it's in near to almost perfect condition. I want to buy it, but I have a phone with a plan. So I want to know if I can still use things like the AppStore or iTunes and things like that. Can I use the Internet when connected with Wi-Fi?

You should be able to do anything that doesn't involve cellular or 3G service.

Maybe you should ask if you could still you a stolen iPhone.