...take exception...a well-executed resto-mod is not respected...
Since you cannot consider us here at the forum the same as an audience at an auction, most of the resto-mods (if that's the proper word for it) commented on here are, frankly picked apart like vultures do carrion, and laughed at vociferously. That's the no respect part, the Pagoda resto-mods like Rodney Dangerfield. It's not "us" per se, but what the collective Mercedes-Benz community believes. Since we're not bidding to buy, this transcends the value option.
Go to any kind of "car show" where American Iron is the standard, and you'll see that except for some rare cars (Some early Shelby Mustang Cobras, Plymouth Superbird, or other certain rare muscle cars) modification is celebrated and valued. It just isn't in general for Mercedes-Benz--and that's not "my opinion" but merely observation of fact.
I pointed to Motorwerks, a local shop, and the engine transplants done by the owner. In the
W126 group, these kinds of modifications are celebrated--but that's a small group. The respect for mods is considerably less so on Gull Wings, 190SL and Pagoda. Yes, there are those such as Mechatronix, Hatch & Sons, etc. that have done some very well executed engine transplant mods, but they are done generally by demand of a specific owner--not because the market is calling for it or it increases the value of the car.
I mentioned the V12 placed into the late model SLK. There are nearly 40 various ECU's on the running gear of the donor SL600 or recipient SLK. Many of these are keyed to work together, and some are also keyed to the VIN...start separating this "system" and now you can see the issue as you start to cobble these things together. Satish gave a presentation on this conversion a couple of years ago, and it was described as something akin to the old game of "Whack a Mole". An example: you put things together, and the car won't start. So, you modify some things, and the car starts. YAY, but it won't get above idle. So, you modify more, and now you get it to run well, but the directionals don't work, and the radio is only on FM. So you fix those, and now the transmission won't shift...these are the kinds of issues that electronics present themselves with. Some of the solutions employed on that mod included having to replace certain ECU's and have them keyed by MB to the new VIN. Others could be re-programmed with the right tools. This is the kind of challenge that some relish, and if you have the patience and time for it, and the end result will be what you want, there's no reason not to do it.
My overriding advice is to speak with others who have done it so you at least go into it knowing what some of the challenges are and where they might present themselves.