Id say you shouldnt pay more than 350 for it. You can buy used Glocks for that all day long at a couple of places.
For a test I would run a fresh snap cap through it to make sure it feeds, fires, and extracts well. I would then check the frame rails to check the level of wear. Then look at the barrel to see how good the rifling is. Other than that just give it a good lookover and make sure nothing looks out of order.