Last couple double rifles I examined were .30-40 Kraig by LeForgeron and Birmingham made .303 English with highly regarded Scottish name on the barrels (can't remember the name either Henry or Frazer). Neither had means of scope mounting. The former was $9,000 and latter $14,000. There are a lot of AF admirers in USA and fact that firm still exists helps a great deal. This was few years ago and not being interested in double rifles I do not know what this would bring in today.
The 60s vintage B Nickel is still usable scope. There are plenty of old German Weatherby scopes still in use. I had 2,5x fixed power B Nickel from mid 70s on my .375H&H (that was when I was foolish enough to buy something so large) and the scope was ok though much, much worse than something like 1,75-5x Vortex Diamondback.
With respect to loads I would think you need to reduce average gas pressure of the standard modern .30-06 cartridge by about 40-50MPa. If you don't reload using reduced recoil (lower velocity) loads may accomplish this. I don't know.
