I thought I would post an update now that I have had 80-90% of the planned work done, and the car back in my possession!
As mentioned, I decided to go with new interior vinyl kits from GermanAutoTops - very glad I did! The seats and doors look and feel great... I will save the dashboard coverings for another project that might involve pulling the dash.
I had two issues with the kit supplied from GAT - firstly the coverings they provide for the door-mounted armrests didn't seem right, they came with a much more prominent seam and left a lot of excess material that needs to be tidied up (photo attached). I am having my guy unstitch and re-do the seams correctly, and trim the excess. Secondly, I seem to have only been sent one headrest cover instead of two. The bigger issue is that it is now difficult to get hold of GAT via phone and I am greeted by a voice mailbox - presumably a result of the current merger with GAHH.
There was some slight miscommunication with the shop on the thermal/sound insulation, there had been a lot of discussion back and forth and I suppose I wasn't clear enough on the final instruction. My intention was to keep the metal floors as original as possible, and add mass-loaded vinyl (MLV) mats below the seats, on the forward floor pans and in the rear bench wells to provide better sound deadening (thank you thelews and 66andblue for your previous posts on this). The benefit of this approach being that the MLV mats are easy to cut-to-shape, and can be laid beneath the removable carpets without any permanent adhesion/attachment to the metal floors.
The shop actually ended up pulling and scraping off the original insulation from the floors, which was showing some signs of degradation, and replaced it with Dynamat, stuck down beneath the seats and on the floor pans leading up to the firewall. The MLV pads were cut to shape and laid on top of the dynamat. Not quite according to plan, but I have to say the overall effect has been great. Far less road noise and vibrations, less rattling, at 60mph+ I can actually have a (loud) conversation and hear the radio!
I am a little conflicted as to whether the dynamat addition will detract from the car's 'originality' for a future owner - but ultimately, it has enhanced the experience for me and I bought this car to drive. A few other bits left to complete, but making good progress.
Some photos attached.