1) you'll regret getting a kepler gpu, nvidia apparently has dropped all support for them, with r9-280x/hd 7970 cards coming within spitting distance of the performance of the gtx780 (in some games trumping it); furthermore 2gb vram just won't do for modern games in 1080p and ultra settings.
2) if you buy a used gpu assume it's in the same condition as your GTX660 1 month ago. because that's the point people realize thier gpu is dying, it's time to get a new one, and OH YEAH, it still works well enough i can Sell it! in short, if you're expecting it to last more then a couple of months your expectations are way too high.
3) it sucks to be without a gpu i know, but i would almost suck it up till i had 100 more and get a brand new gtx...