I'd say offhand that any car expected to run in the anticipated service would almost have to be tubed and a fully-appropriate interior designed and put in. No one can be expected to pay the expected price consistently to ride in tired old cramped ex-Pullman-style accommodations -- there might be a little nostalgia, or sense of historic participation (like the American Orient Express expected to see), but the joy of North-by-Northwest-style accommodation won't last, especially if there is any poor riding or curving compliance problem anywhere during the night.
A trend in hotels for decades is that older-style rooms are no longer particularly attractive; even as early as the Traymore demolition, one of the great architectural crimes of the 20th Century, it was understood that no amount of cost-effective work could make it a paying proposition. This new train has to be at least as good as a good hotel, possibly as good as a good resort hotel or cruise ship. You won't get that by refurbishing cars from Ozark Rail or AAPRCO.