I don't think you can find a finer series of cars than the W126 series. The 300 gas gets decent mpg, but not as good as the diesel. The 560 has more poop than the 420, but is potentially more expensive to repair.
You say you have a Camry: all but the diesel should outperform the Camry with no sweat, especially above 90 mph.
For what a mid-range new automatic Camry SE costs ($20K here in Portland) you should be able to purchase a fine W126 series car & have quite the bank account/repair & maintenance fund left: whether it would last +100K miles or 10 years is open to debate and probably a factor of how good the car really was to begin with... Good records should command a premium, but a cheap car that looks good and runs good may be a real find. When you buy a 12-17 year-old car there are really NO GUARANTEES, are there?
Air conditioning/climate control refurbishment, an engine top end & a transmission rebuild could eat that fund up in 6 months, but this would be a worst-case scenerio in Spades.
On the other hand, I bought a '91420SEL over two years ago and have spent less than $1,000 for a few VERY minor repairs and replacements of normal wear items (brake pads), all scheduled regular preventative maintenance (and then some!), a set of chromed factory wheels and studded tires in about 25K wonderful & troublefree miles.
It flew through D.E.Q., is reasonable to insure, my wife gets 20 mpg on her commute, I've carried 3 passengers and all our luggage up & down the mountains and to the coast in comfort.
I fully expect it will make it to 200K without major problems, but if it needs something done... Just do it.
One less thing to worry about. Worry more about somebody hitting you and having to find a replacement.
What will it be worth in ten years? Who knows? If it is in as good condition as it is now I'll just drive it for 10 more.