As far as I know it does have a warranty, but not sure if he still has the box for it, not sure if it can be returned without it or changed if it fails.
It is clean, the rig was mainly used in his home, we actually set them up together.
I have seen it mining, haven't seen it actually used for gaming or anything besides mining.
What software do you suggest using for stress testing? Or do you mean just run some games on it?