"He thinks the engine may have been permanently damaged by too rich fuel flow and feels the best bet would be a rebuilt engine at $28K. "
It's a good bet all right - for him.
He wants you to spend $28K without even running the standard checks like compression and leak down, looking at the cylinder bores, testing the oil for fuel contamination ...
I would get the car to another mechanic as quickly as possible. And I would certainly raise hell with Mercedes about their flagship model needing a wiring harness at 55K miles.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe