Hello,
I am currently driving a '99 Deville 50th anniversary edition. This is a car I purchased from the original owner with 58k miles in April 2008. The car now has 75k miles. As far as I am concerned, the Northstar is good when it is good, and when it goes bad, hold on. They are expensive to diagnose and fix problems. My last northstar was a 98 deville that blew up at 151k miles. A month later, my friends northstar in a '01 deville went at 155k mi. I would have not bought my 99 except for the fact that the car has been my grandfather's since new. I guess these are respectable lifetimes for todays cars, but I was more partial to the 96 Fleetwood Brougham I had that would go 400k taken care of. The long and the short of it is, If you are not putting many miles on a year, and take great care of your car, these are great cars. If there are no service engine lights on, I would say you are fine. In Todays market, this cars price is falling rapidly. Be sure not to overpay for this car.
Dave