Even with SLI, your system would draw less than 500w, so you don't need 850w. If I do my dollar conversion correctly, that is a pretty good price if it includes the warranty. If you don't know anything else about the psu (if you're buying it online from a stranger), I suggest you may be better off with new.
Thanks for the advice guys. I'll steer clear of that then. I took the case from him though which was a bargain. Its the cooler master elite in mint condition for £35. Offered him £30 for the PSU but he said no. Thanks again guys.