I can tell you that the stock oil pressure guage on the 4G63 always runs low. As a matter of fact I know many guys whose warning light comes on when they are sitting in traffic but it increases under normal driving - it's actually a common thing with these cars. They stock psi for the oil guage is 11.4lbs at idle - not much. What you should do is get a oil pressure guage and unthread the OEM one (it's right next to the filter by the oil pan and see what it's running at on an actual guage.

On the other hand, with the smoking, rust and cars history (replace motor, bent valves) your probably better off passing this one up.