I'd probably pass on repairing the frame. I suspect there is a market for the front fork as their fairly unique - my MS Racing bike came with a cheap aftermarket fork and if I was restoring that bike I'd totally want a proper MS Racing / Alpinestars fork to make it more period correct.
Well, that stinks about the fork. Otherwise a very nice looking bike. If you want to keep the DX drivetrain and have shifting on your bars other options include 1. Gevenalle or IRD brake levers adapted to take old downtube / bar end shifters (I'm running the IRD ones on one of my gravel bikes)...
When I saw this I thought it was a Rocky Mountain or a Talbot from Vancouver. Did Derek get started at Holdsworth or did he move back to the UK after his Rocky Mountain days?
I live north of Seattle in Vancouver and its the same here which is why I've been highly motivated to modify my vintage bikes with better climbing gear. Doesn't help that I'm fat and out of shape.
What I've noticed since joining the Retrobike.uk site is that Europeans have access to a much deeper supply of vintage bicycles and components than we do in North America. And I feel another difference is that in North America there is a bias towards a quality new replacement part if available...
I feel that Jan Heine would be happy to see this bike restored and being used again - I mean he loves the original Rene Herse bikes so much he bought the brand to continue it. He supports classic Herse restoration here...
My preference is wedge stems over expander - i find the expanders sieze more frequently. I also just watched a video by the BikeFarmer which suggested that some expander type stems can fail.