How long will a Northstar last?

[deerefeed]

I've seen a few late nineties Devilles for sale with over a half million kilometers (300,000 miles). These are selling for a few thousand dollars and look appealing provided they last for a few hundred thousand kilometers. I'm assuming these were driven primarily on the highway. I guess the mindset is that highway driven cars are in better shape despite their mileage.
Are the repair costs much higher than say, a Buick or Olds. I currently drive a 1988 Buick Park Ave with 400,000 km (275,000km on the second engine) and it has been quite reliable but after 18 years of Canadian winters, the body is starting to show its age.