Pull the pan. By the time you go through all the "easy" stuff you won't have any engine left. The fact that the cam went south for the PO and he didn't pull the pan is a red flag.
+1, The previous owner already told you the engine ate a cam, so, all that metal clogged his filter, it bypassed due to the clog sending unfiltered dirty oil through your engine. Your oil pump took the first hit, it probably looks like it's been sand blasted internally. The metal filings have your oil pump pressure valve sticking causing your pressure variations. Before running the engine any more drop the pan, replace the pump, check your main and rod bearings to see how much damage they took. You may get lucky and find the bearings aren't too bad but I'd say they received some damage too.
