The most number of miles I have seen on one of those is 693K on a Crown Victoria, your car will be around for a quite awhile.
I was driving 1970s cars well into the 1990s, and many of them had far more than 100,000 miles (in the 150-200K range). So yes, those old cars normally lasted far more than 100,000 miles. It's just that before the Reagan era middle class people wouldn't dream of keeping a car that long. Now many of them must.
I agree with you that your Buick is far nicer than anything offered in the same range today. But I was just making a point about the crash in the standard of living of the American worker. It most definitely effects the car market.
I just bought my 98 Lincoln Town Car Signature Series, and it has 236,000 and it runs great. I bought it like 2 days ago, and drove it for about 6 hours, and I had one of my buddies plug a computer into it, and it came up with misfire. Could this just be bad spark plugs, or what would I do?
When you first buy any car, check the regular wear and tear parts, and tune-up related items. If they look original or of questionable age, replace them. This will save you headaches later on...
Of course it could be a bad spark plug, or a bad wire, or a bad rotor, or something else.
I would get a spray bottle with water, and when it is really dark, get under the hood. While the engine runs, I would spray water all over and look for sparks flying off the ignition wires and between them.
Next step, I would take out the plugs and inspect them closely. If any one looks suspicious, I'd replace all of them including wires, cap and rotor. Or if you have a coil pack, inspect the towers and electrodes inside the towers for any corrosion, cracks and deposits. When in doubt, replace them.