It is 2013. Is it feasible to purchase an iPhone 3GS for telephony and skype? I'm thinking about getting an iPhone for a friend whose vision is decaying because of the possibility of making it display large text.

However, the budget is very tight, buying a new iPhone is out of the question. She needs only basic services: telephony, the occasional text message, and Skype.

Would a used iPhone 3GS make sense for this? According to GSMArena, the 3GS is still supported by iOS up to version 6, which (so I understand) makes Skype compatibility no problem. Is there anything else that might speak against getting her this 4-years-old model?
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).