1997 Cadillac Deville Engine replace or repair?

Engine Mechanical problem
1997 Cadillac Deville Automatic 140000 miles

Low oil pressure light has been coming on when the engine is warm (operating for 20 min or more) and at idle speed. The warmer the engine is, the higher the rpm at which the light comes on. Cadillac dealer tells me engine needs replace or rebuild. 1. Should I get a second opinion? 2. Dealer's recommending a used engine (84K miles and he'll give me a year warranty) - pros and cons of this versus rebuild? 3. What should I expect to pay.
I really like the car and would like to keep it for a long time.