I have a very good looking 1984 300CD. If you look closely, there are some rust bubbles at the lower parts of the body panels and holes in the floor pans. I have located a donor car and I am considering taking the floor pans and lower body panels from the donor and eliminating the rust that way. The car, engine and transmission are very strong on this car and I'd really like to preserve it. How do-able is this?

I am curious as to what others are going to answer on this because these cars have no frame and are sort of like an air craft fuselage in that the whole of the structure gives the Car strength. Removing that much metal even if you did one side at a time I am not sure how you would support the Body to keep everything lined up.

Then of course there is how good are you with a Welder. (in the 1960's as a teen I had worked part time as an arc welder and since that time gotten very out of practice. But, welding the cross member required me to buy a flux-core welder. I had never used one before and even when I arch welded I was extremely no good at over head welding. I did a lot of welding and grinding and re-welding and grinding and so learning to use the flux-core welder.)

That being said I welded up a cracked rear cross member and added plates to strengthen it and some vilified me for that telling me I should have junked the Car.

If it was me I would just patch up the floor.

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84 300D, 82 Volvo 244Gl Diesel

The main issue is the rust proofing and sound proofing that mercedes uses. That all needs to be ground off before welding and its a smelly smokey process. Even if you do remove it, it often ignites when you begin welding so you have to constantly stop and put out the car before it gets fully involved.
Full disclosure, I have been patching rockers and pans today in an 84 so my head hurts from the undercoating smoke.
It is doable and the car doesn't fall apart when the main members are cut. Just don't drive it that way.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

I was not saying it would. It was more of a reminder to keep things well supported and lined up.

That sound proofing/rust coating also makes it difficult to pop a dent out from the other side accurately with out removing it. I am speaking of inside of the trunk and the Doors.

Is there any chemical treatment that softens it enough to scrape it off?

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84 300D, 82 Volvo 244Gl Diesel

Its a vinyl like compound. 3m stripping wheel, and an endless supply of drill batteries is how i do it. I cut out major sections of one floor pan and rockers, but welded small patches in anything under the size of a sheet of paper. I bought replacement pans online. Aftermarket, not MB. The rockers i had to fab sections myself.