Why the harsh/rude response?
The OP asks a perfectly legitimate question.
Mechatronix are indeed well known for this type of work but that doesn't mean no-one else is capable.
Not a rude response but reality. You know full well, Stick, that the cost of a full “Concours quality restoration” is not a small fortune but a large one. Then add an AMG engine on top of the restoration? Does MB make any AMG engine in crate form (you know like many American companies?) So those two requirements send the price into orbit.
The OP mentioned Mechatronik. So now there is a known high level of quality standard added. They are very good at what they do. I saw and drove one of their cars. Fantastic. Superb quality. Drove like a dream. I’m a big believer in trusting the experienced. So if you want Mechatronic quality why not just start there?
And Bob Hyatt agrees—why not just go to a known entity?
If you know of other shops capable of doing this kind of high-quality restoration with an engine swap— somewhat close to the Mechatronic quality, let everyone know. Is this a task you could take on? Is it a task you would take on? Have you done similar engine swaps?