I never diagnose in front of the PO unless it will lower the price. Look at what it will need in addition to an engine and decide if it is a good deal. Now consider the addition of an engine. Still a good deal? $500 is ~ scrap value. The first thing to do is diagnose the rough running - after you have it safely home. I'd buy it unless it is trashed or I didn't have room or want to fool with it.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.