Oil Pump?

I am looking to buy a 91 GST. The owner tells me that it's got an oil pump problem, other than that, it runs fine. After talking to a mechanic, I'm beginning to think that it may not be just an oil pump. Before running any diagnostics or compression tests, is there anyway to find what other problems are wrong with it? I 've had 2 people tell me that oil pump problems are not common in DSMs. I haven't seen the car yet, so I can't tell if there are any problems yet.