Date registered: Jan 2006
Vehicle: 73 280SE 4.5, 71 300SEL, 03 BMW Z4, 72 Fiat 850, 80 Triumph TR7, 85 Porsche 911
Location: DC Area
Mentioned: 1 Post(s)
Quoted: 95 Post(s)
You might consider an American engine too. I have intentions toward a Chevy small block getting installed in my 109. Another option is a Ford 5.0. Both companies have engine management computers and wiring for standalone swaps like this. If you go to a manual tranny (another kettle of fish in modding the engine bay and firewall), wiring will be even that much easier. If you get into Hotrod Magazine, or any of their competitors, you'll see adverts by the boatload for standalone engine management suites, oil pan modification outfits (the Chevy's will require a mod to fit the Mercedes crossmember. The Ford's is much easier), front end replacement, replacement brake systems, etc etc etc. I like the option of an American engine swap because at some later point, I can add a supercharger or turbo, and there's tons of support, parts, knowledge, etc. As Drew mentioned, Mercedes is much more highly specialized, and the knowledge base is thinner.
Regarding speedo and such, an option is to to "retro modern." There's an outfit that will take your speedo cluster and replace the internals with electronic bits. They can use the same face plate(s) or make everything one big LCD screen, with "analog" gages, or digital. Look for Dakota Digital on the internet - they do all kinds of custom work.
If you use what you already have, but drop in a different rear end, change tire sizes, etc, any speedo shop can regear your speedo internals to account for the changes.
If you decide to go American and want to replace the fewest parts on the W111, you might weld the American driveshaft to the Mercedes one and leave the rear end in. My dad swapped a Ford 289 (Mustang engine) into his 6 cylindered 108 and left the rear end alone. There's been no issue, even decades later, with the power and torque upgrade. I don't know how stout the 111's rear end is, but I know that the 108's and 111's are directly interchangeable.
Note though, that any mod you do to your car can change its value, especially if you put an American engine into the thing. I'm by no means a purist, and would actually look for such a swap having already been done...
Regarding rust, as long as the car hasn't dissolved under its own weight, everything is replaceable, using either a donor 111, or metal from your local hardware store. If you can cut and can weld, such a repair isn't within the realm of "experts."