Yeah, .... no.
360 fats, even the various later revisions, were all fairly unreliable. The causes are complex and sort of irrelevant here, but suffice to say that the risk isn't worth it. To make things worse, with a used one it might be one that failed and was already repaired, and those usually re-fail within a much shorter timeframe than the original failure.
If you want 360, get a slim for sure.
Are you just talking about playing through the SP games and not interested in MP? Buying a used boxed 360 slim, playing it, then reselling it shouldn't lose much $ for you as long as you sell it before the XB1 hits. When the XB1 hits, 360 prices are going to hit the floor (a la PS2 after PS3 launch).