It is an old car it won't be perfect, I know of a few "perfect" W126's that sell in the mid to low teens. Those cars are know to have weak valve stem seals, not an expensive diy repair. A quart of oil every 600 or 800 miles is within spec for that motor. Have a shop do a leak down test, as long as the oil was changed I doubt the bottom end is going south. If the body is clean and the interior is nice, another $3k and a little work and you will have a real nice ride. For $4k it sounds about right.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200