A real good friend of mine did this with an 89 244 he owned for several years.
The car came out just fine. I did not see it before the finished product was done, so I do not know exactly where he cut the roof- I just know that a factory roof was sectioned into the roof.
It's cool but a lot of work. An aftermarket roof is probably a whole lot easier (since it doesn't usually involve any welding, or painting). But if done by a competent welder, it shouldn't affect the integrity of the car.

There isn't any EASY way to do this. The sunroof cars and non-sunroof cars are quite different upstairs.

With a good donor car available, any body shop should be able to handle this work. My thought is that the best way would be to cut a big hole in the recipient car's roof, and use a "Bead roller" tool (www.eastwoodco.com) to make a neat edge all the way around, and trim the donor roof to match, then weld all the way around it. You'd need to use some kind of reference point or else you're not likely to ever get a factory headliner to fit in there. There are a lot of layers up in the windshield header and A-pillars. Cutting through that area is quite difficult (and a whole 'nother story!)

Note large hole in roof!
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K