I might buy a 1996 GMC YUKON the converter has a hole in it is that hard to fix and what would it cost around?

You need to replace it. The hole probably means it's rotted out from the inside. You can get a fairly cheap generic converter, but you'd probably have to weld on a fitting for the rear O2 sensor. A muffler shop could do the whole job for $300, but they'll try to talk you into spending twice that much.