Real question to ask yourself is where you want to end up?
If its the same car and $10,000s incrementally to accomplish a 15 or 20 or 30% improvement in aesthetics / driving experience; you would certainly be better off selling your car and putting the proceeds and "restoration money" towards an already restored car.
Alternatively, ask yourself what your "all-in cost money" can buy you; you have had the SL for 25yrs, maybe you want something else?:
Sell you car: $40,000
Restoration money: $20,000
Unknown repair/Time-lost/etc: 5,000
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$65,000
$65,000 can buy a driver quality late 60s Jag XK-E Roadster, 60s Corvette, early 70s 911, or 80s Ferrari 308 QV.
All cars that may seem out of reach, but are equal to a Pagoda all-in cost "restoration".