Worn camshafts are another problem. Browse the b5 FAQ, it has a new owner checklist at the top for common problems.
Since the oil pressure warning light is already on, expect to change the camshaft. A bad turbo is also possible. The crankshaft main bearings are also suspect but if you're already changing the oil pump module it's not that bad (except that you have to remove the transmission to get the crank off). Plan to put a few thousand into the car.