Radeon 7970,7990 and even a 6990 would be great if you can find them in good condition.
There are tons of Antminer S7s on ebay that include the PSU which are great deals in your situation.
Or you can even get an S5 if you can find them for ~100 bucks, much more rare though.
I wouldn't recommend him 6990 or s5. I mean something like 7990 or s7 is going to be pretty hard to sell after he's done with it, going for even older hardware would be just stupid at this point
A 6990 easily hits 700 H/s on XMR, which right now is 0.078 XMR, or $1 daily.
The S5 also makes $0.7 of BTC daily (though I suppose that will go down unless price goes up)
OP has free electricity, so he'd have a relatively quick ROI depending on the deal he finds. There probably won't even be a need to sell these.