I'd buy a W126 Series S-Class
The nicest, cleanest, low-mileage one you can find!
One with great maintenance records: have it inspected by a reputable shop before you buy it. Drive it for a couple hundred thousand miles.
Go ahead! Pay a premium price for it. They're well worth it.
Take all the rest of the dough you would have had to originally spend for a later model S-Class and see how many years (decades?) worth of service and repairs that covers. Then, when that runs out, you can enjoy how much less expensive it is to have a W126 car worked on vs. what you would have spent on one of the newer ones.
'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435