I am guessing here, but if you have ever priced a used engine for the 280SE you can see where an engine swap is something to think about.

A six from a 280SE goes used for about $5,000. I think you can get a used 4.5 for around $1,000 but the early ones might be hard to find and more expensive.

And when you consider you could pick up a wrecked 300D for $500 and everything you need (almost) is right there......

I am betting this will be quite slow to drive. Let us know what type of MPG you squeeze out of it.