I would buy an otherwise nice double in .458 only if I was certain that there was enough meat in the barrels to rechamber to .500/.450 or .450 No. 2 - and the rifle would need to be really cheap. I would have it re-proved in England.

Same would apply to a rifle that had been rechambered but not re-proved. Depends on price, quality of the work, and how much sense the project made to begin with. I would still have it re-proved. Double rifles aren't bolt rifles.

I wouldn't buy a gunsmith-built DR on a shotgun action.

The last question is a little narrow, but no. To me, double rifles of good original quality that have been re-barreled or re-chambered to a desireable caliber, and properly re-proved, are never worth as much as a double of equal quality in the same caliber with original barrels in excellent condition - not even close. If they haven't been re-proved, the price drops some more, and would have to be low enough to justify the risk of proof failure.
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