I guess if it a complete machine and did not fall off of a truck or something, then for 50 bucks you could hardly go wrong. It's worth almost that in scrap value.
I'd be surprised if it actually was a surface grinder. I looked for over 2 years to find 6 X 12 surface grinder in decent shape. I ended up shelling out a lot more than $50.00 when I finally found one that fit the bill. If it really is a surface grinder and is in reasonable condition it'll be the least expensive one I've ever heard of.