autto.net

for example like maybe 180'000 to 280'000.

You couldn't TOW a new Buick 180,000 miles without something breaking. More like 125,000 before real problems happen.

If its got a 3800 I think you will be in that 280 range. They had a few minor issues with the intakes but they are pretty cheap to fix as long as you dont ignore all the signs and keep trying to run it till it completely dies. Just look at the ads for used cars, you will see plenty of ads for 3800 equipped cars that are in the 200K range that say runs great.