OP, if you want a 13" screen then I actually think the MBA is the way to go. If you buy or refurb from Apple then you shouldn't need to worry about freezing issues. Or on top of that go on Amazon and pickup an AppleCare.
For most uses I actually think the 13" MBA is a better choice than the 13" MBP because the Air has higher native resolution which is a big deal to me.
With all that said, $400 is too low. We got more than that for my wife's 3 year old White MacBook on eBay. Mac products hold their value very well. It's great when you want to resell yours and upgrade, but it makes it a downer if you want to "break into Apple" on the cheap. But remember, you typically get what you pay for.