"bargain" 900 or "excellent" 850?

My first "real" camera was a Sony NEX-5R, purchased not that long ago. Since then I've been trying to absorb as much knowledge as I can, and have taken as many practice shots as I can squeeze into each day. I'm scheduled for a photography class in a few weeks, too.

But I'm already feeling the pull of full-frame, and I've just about talked myself into getting one. I don't want to invest too much money before I really know what I'm doing, or I'd go with the a99. Plus I'm curious about the FF mirrorless we may see next year.
So in the meantime I was looking around KEH.com, and I found a couple of interesting options:

- A "Bargain" a900 for 1169.

- An "Excellent" a850 for 1219.

The prices are practically a tie, but I have no experience with KEH and its "Bargain" criteria. Is it worth the risk of the camera's condition to get the higher specs of the 900? Or should I just take the known quantity of the 850? Or sell a kidney and upgrade to the 99? Or just save the money and buy some kind of crazy expensive Zeiss lens for the NEX instead?