Reply to Buying used macbook, how old is too old?

I'm going to be buying either a 13" MB Air 13"-15" MBP within the next few days.
I'll definitely be getting one with 4gb RAM, and that can be upgraded to the latest OSX

My budget is about $700, and I will be using the computer only for TP2. There are a bunch used in this price range, all over the place in age.

My question is, how old is too old when buying a used Macbook?

Also,
Should I spend the little extra to get a 13" Air, or stick with the Pro?