Good buy on the rifle and a great cartridge for anything you are likely to use it for on the North American continent. There is an ample supply of factory ammo available, and I recall the Krieghoff is regulated for the 285gr Norma loading, an easy matter to find at most on line sites. I echo the comments on lighter loading, I'd work backwards gradually from the regulated load. I have done that in several 9.3X74 doubles I own (SXS and O/U) and you get to the point where you can perdict the dispersion at various ranges.

Several of my 9.3's are scoped as well. I believe that Krieghoff has mounts in stock and an in house gunsmith who will mount them for you. I have spoken to him, and he seems very astute on these issues. I added mounts to a few of my doubles, but since this is something that has to be done right to maintain even a shred of accuracy, I'd go with someone who knows what he is doing rather than a local smith. If Krieghoff can't help you, JJ at Champlain is the route I'd take. He is backed up on work right now (what good smith isn't?) but he is surely worth the wait.

Congrats and have fun. Dave