I’ve gained so much from this forum and given very little in return. The pictures are post-op as I didn’t plan documenting it but I think they’ll suffice. So here’s to paying it forward for what it’s worth.
If your lower door seal retainer channels haven’t rotted off yet consider yourself lucky. If they have this is a solid repair that doesn’t require welding and your new door seal will fit like new. An argument could be made for just gluing the seal in place which was my original plan but after cleaning up the door it became evident the retainer was more necessity than over-engineering. In my experience... it’s better not to question Mercedes too much even for mundane door seal retainers but that’s just me.

I was dreading this repair. I’m not fond of body work but it was a pretty easy job in hindsight. The seal retainer channels are spot welded. Mine was rusted completely through at the aft edge barely hanging on midway down, but probably wouldn’t support 3 oz. of foam rubber. I peeled it off by hand until the last spot weld. It almost popped by itself when it saw me with a flat blade screwdriver.
The door itself was in pretty good shape but I cleaned up all the rust down to bare metal. Ospho®️, chip brush, and Rustoleum because that’s what I had on hand.

Bottom aft corner showing the intact side and the repainted lower edge with channeling removed. I measured 7/16” tall and 1/2” from where it is welded to the door. The retainer lip is about 3/16”.

[Image 1]

As it turned out I had some aluminum channel stock for 1/2” plywood edging. That inspired the solution but it was too big. The local hardware store had 3/8” and it was near enough to my measurements. You’ll need at least 30” in length. I found them sold in 4’ sections.

[Image 2]

[Image 3]

The stock needs to be trimmed on one side to achieve the same cross section for the retainer lip as the original. I thought this would be a piece of cake. I had a nice little jig made and with my new Bosch rotary tool and a cut-off wheel it looked like a little table saw. It wasn’t cake. The mandrel kept holing the thin discs. I ended up using a vise and my Bosch oscillating saw which worked pretty well. There’s probably an easier way so don’t let this shy you away. I don’t but maybe you do, have patience with a hacksaw. Be sure to file off the burrs from your lip to avoid getting blood all over your fresh paint.
It’s all down hill from here.

[Image 4]

I bedded the stock in a polyurethane sealant and secured it in place with aluminum pop rivet fasteners. Again, because that’s what I had. There are some good adhesives that would probably make sense to avoid fasteners altogether. The only problem was I had none.

Notes: Instead of butting the stock against the doors edge, I used the scrap I just removed as a spacer to achieve the same dimensions as the original. The rubber seal would still squeeze in there if you butt the stock closer but it might elongate the seal more than desirable.
Coat the inside of the aluminum stock with silicone grease. This eases installation but more importantly protests the aluminum stock from Corrosion.(carbon rubber is not friendly to aluminum when water is in the mix)

If you decide to fasten with screws you can probably skip this step but the head of my rivet tool needed more clearance so I had to cut notches for each rivet. I used five 5/32” x 3/8” rivets.

[Image 5]

The seal installed snug and secure which [Image 6] shows but won’t let me attach more than 5.

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I guess I’m not acquainted with how to post a write up. Here’s the results.

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I just found the DIY link to save 10% which explains all.