Alright here's an update on the update.
The customer and I huddled a bit and decided that since I was going through all the effort to rehab the missing floor, it didn't make sense to leave the dreadful original seats in there, since you could barely sit on them.
This should give you an idea of what they were like:
So, keeping in mind that THIS IS NOT A RESTORATION, I hatched a plan to update the entire interior in a manner that was within budget, mostly Mercedes and tastefully period appropriate...ish. I have the original grey MB-tex seats from my '90 300D left over from when I converted to leather and they're in nearly perfect condition. I'm going to weld the attachment points for the 110 seat hinges to the seat backs, and it turns out that the bosses already exist in the bottom cushions. I'll use the nicer 108 slider assemblies welded to 110 pedestals. I will be cutting new door cards from bath board, with a nice thin cushion of foam overlaid with two parts grey vinyl on top and a nifty '50's/'60's themed silver-grey fabric. These two sections will be bisected by a thin curved piece of birch wood strip...shhhh, don't tell anyone, but they're from IKEA wood blinds we had lying around. My inspiration for this door card combo is the early '60's VW Type 3 interior:
The dash wood appears to be beech, so I hope I can come up with a stain and finish that makes the two types of wood appear similar.
Ok, so now taking off my Serge the interior decorator smock and putting on my Randy the Riveter cap, back to the nasty stuff that started this project. The floor pan rot is even worse than expected. It's extended up into the unibody frame rails on the RF side, so I'll need to box those in, as well as the sections below the passenger seat frame.
The rest of the pan/frame issues are mostly just cut n' patch stitch jobs, followed by rust treatment and a sealer like bedliner or some such product. The rubber floor mats were mostly rotted, so I'd love some advice as to how I can recreate the floor coverings without relying upon Home Depot motorsports, which is kind of tacky. I can do tacky, but it's not very satisfying. Any hints on how I can deal with a sadly dilapidated padded dash would be greatly appreciated. On the bright side, the headliner is in really good condition.