Some answers may be useful:
Chapuis, Verney Carron, Merkel, Zoli, Heym, Krieghoff, Browning, Rizzini and most other makers regulate their doubles from 50 to 75m, depend on makes and if scoped or not.
Most of them will accept a special order for 100m regulation at a price to be defined with customer.
Most of the double are pre-equiped to mount scope or red dot. These are the cut in the rib, filled by a metal piece, you remove to instal claw or EAW or Recknagel or other maker scope mounts. Quick detach one.
Good work is expensive believe it or not, cheap as nothing to do with doubles but the price may vary.
Modern over under are easily scoped and some are designed to use battue scopes or red dots (Blaser, Merkel, Heim, Zoli)from the start.
Some of them are specialised rifles for driven hunts (euro way) with independant cocking.
Some have build in regulation system that allow to adjust when you change bullet weight, make or shape. Not something to fool around every day at the range but effective when you know how to work them.
Most of the big caliber(over 375H&H)SxS are not designed for scope but if you pay and ask for, manufacturers can do it.
About the 30R Blaser: it is a bit more potent than the 30-06 equal to the 300H&H flanged magnum. We use it mostly in driven hunt in double or drilling but also in stalking single shot break open.
Good choice
Dom
