The Jacobite isn't about fine dining at all, because it isn't provided - it's specifically sold as a steam train journey, so I'd say at least some opening windows are important. Mark 3s wouldn't be appropriate at all.
I think there are basically two types of charter:
1. A journey where the locomotive (be that steam or diesel) is the thing of interest. With this type comfort is of low consideration, but being able to hear the locomotive and smell the smoke is important - hence opening windows.
2. A journey where the service (e.g. fine dining) or scenery are the thing of interest. With this type, aircon, large windows and high seat comfort are fairly valuable, making Mk3 FOs like the ones used on the S&C very appropriate.
You do get a bit of an overlap (e.g. being steam hauled along the scenic WHL extension) of course.
For the former it probably still makes sense to keep "Trigger's broom"-ing Mk1s and early Mk2s, including fitting CETs, anti-override and door locking if that is needed, and potentially modifying windows so you can't stick things out of them, e.g. by adding a mesh or bars over them and sealing the droplights (Mk1s already have internal handles). The approach to wheelchair access needn't be the same as the mainline because staffing is very high; one option is to use BFOs or BSOs with access cut from the van area into a dedicated area with PRM toilet added. Mk1 coaches are a very simple thing and can essentially be repaired forever.
For the latter, if you were going to build new, Swiss-style panoramic coaches would be a good choice.