- Does the engine I source have to be the exact same variant of the M119?
- Would it be possible to put a later variant with coil-over ignition into the car? (I always hated the distributors)
- Lastly, 5.0L variants seem to be much more plentiful near me, are there any inherent problems with putting a 5.0 in the car other than having to swap the computers?
1) A replacement engine must come from either a W124 or R129 to be plug+play; specifically it must be an M119.97x engine with distributors and LH fuel injection. The 1990-92 500SL engine is M119.960 and WILL NOT work without major downgrades. And, a W140 donor will require swapping the entire oil pan/pump and exhaust manifolds... more hassle than it's worth.
2) It's possible to use the later coil-over ignition engines, but you'd have to ditch all factory engine management and use standalone (Megasquirt, etc)... I wouldn't recommend this as it's a ton of work just to lose distributors, and there's not much gained. Your mechanic probably wouldn't touch this with an eleven-foot pole.
3) The 5.0L swap is awesome and highly recommended. But you'll want one from an R129, specifically a 1993-1995 500SL/SL500. You'll need a 5.0L LH module and EZL, but that's it. Oh and you may need to pull the first-gear upshift relay; this was discussed in old threads where people did the 5.0L upgrade on a 400E420.
If you do this, plan on a lot of "while you're in there" maintenance... a number of jobs are super easy with the engine out. Like replacing ALL the power steering hoses, pulling the steering box to re-seal the output shaft, replacing the idler arm bushings (spec is 0.5mm max play), new engine mounts, and degreasing the engine compartment. Don't forget you also need to pull the valve covers & oil pan off the new/donor engine, may need to replace upper chain rails, and definitely replace front+rear main seals. I'd also re-seal the front head covers (what the distributors attach to), new valve covers, etc etc. Starts adding up fast, but when you're done you shouldn't have to touch it for a long time.
If you are paying a shop to do all this work... it's going to be EXPENSIVE. That $2k figure is likely just labor to R&R, without touching anything else. You could save some $$$ by buying the spare engine and doing the prep work on the donor 5.0 engine at home, then supplying the motor to your mechanic ready to swap in. The replacement seals, etc are not expensive but you'd need to borrow or buy some tools to get the crank seals replaced.