Looking for an inexpensive MacBook Air

Hey Core, my wife would like a MacBook Air, but her needs are extremely limited. She's been getting by with an iPad as her only computer for a couple of years and she doesn't really need any more capability than that, but she'd rather have a laptop form factor.

Since she doesn't need the latest specs, I thought it might be smart to buy an older model. That said, I don't know much about previous models of MacBook Air. How old is too old? What do I lose by buying an older model?