Komatsu D53-16

I am thinking of having a bid on the above bulldozer. The seller claims it has been running fine and then the oil pressure has suddenly dropped significantly yet there doesn't appear to be any change in how the engine runs, no abnormal noises. I was wondering what things you would consider as the causes of this and how serious and expensive to repair as in full engine rebuild etc. My thoughts were big end bearings though I would have thought they would be accompanied by some noise and would happen over a period of time. Could this be a problem with the oil pump, the pick up or maybe the oil filter.

I would appreciate any thoughts.

Thanks Ken

Komatsu D53-16