Good question with possibly flexible answers.

First off, I wouldn't buy a smithy's bench made DR-on-shotgun action before observing the manufacture of the proof load cartridges and firing of said cartridges -by the smith- himself.

I think I like that idea and will store that one aside for future excercise if the opportunity ever comes up.

I wouldn't pay as much for a sleeved/rebarreled/rechambered for different cartridge gun as I would for a good, in-proof and in-shape original specimen.

Now, as far as rechambering and reproofing goes, there are BPE guns out there that have had thier chambers cut to take modern (available) brass and should never be run with full house nitro loads let alone full proof nitro test loads... that I would happily have in my rifle rack -- and would only run Nitro-For-Black or black powder loads with appropriate weight and designed bullets for said BPE class loadings.
I wouldn't want anything like that sacrificed to full proof test nitro loads.

Get my drift there?

--Tinker