It is 2013. Is it feasible to purchase an iPhone 3GS for telephony and skype?
If you are thinking of doing video Skype then you better go with the iPhone 4 as the 3GS doesn't have a forward facing second camera. 

The iPhone 3GS will likely be somewhat sluggish (depending on what you are used to), but it does go to iOS 6, and is a great phone - especially for calling and texting (although I have seen the keyboard be sluggish, which wouldn't be a problem if they were typing slow).