First of all, the valve tool is not $200, it's around $50, and is available from Assenmacher Specialty Tools. MB dealers are now reselling it.
Second, pulling the intake manifold off is not that big of a deal. The real hassle is getting the injection lines off, which you need to do anyway. While you have the intake off. you can reseal the lower to the upper half and be sure you are starting with good tight vacuum.
The parts to convert to a dual chain are about $250. You should also plan to replace all the slide rails and the tensioner, another hundred or so, and add another hundred for a gasket set.
You will need have a machine shop replace the bent valves, check the guides, do a pressure check and install new seals. Maybe $200 per head.
So if you do both sides, you are looking at 50+250+100+100+200+200=$900, which is probably cheaper than a used engine you could trust.
FWIW, I did not encounter the thread problems when I did my 380SL.
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