I am by no means a ML expert, so take this for what it's worth..
The biggest adjustment for me, going from rifles to ML hunting, is figuring out and mastering the cleaning regimen. With a breach loading rifle using smokeless powder, I have no problem with firing off a few rounds then sticking the rifle in the cabinet for 6 months before cleaning. That's a big no-no for a ML. Also be diligent in the use of anti-seize on the breach plug.
I have a Hawken sidelock, used to have a CVA Buckhorn (inline/non-break barrel), and recently acquired a Traditions G4 (inline break-barrel). The break barrel is SO much easier to operate and keep clean, no special tools required to remove breach plug, insert primer, etc. So for me, the break barrel with tool-free breach plug removal would be first requirement.
As far as propellant, I like BH209 for ease of cleaning. It's more expensive than most other BP substitutes, but you can shoot several times consecutively without cleaning, very convenient during range-time.
One thing with the G4 is that it's extremely lightweight, even with a scope and sling mine weighs around 7 pounds total. Great for packing, not necessarily so great for recoil. The nice thing is that, same as any ML, you can down-load it for range time then bump up the charge for hunting. My technique is generally start at around 100 grains (or equivalent) by volume of charge, then increase until accuracy starts to drop off. After you get your hunting load figured out and scope zeroed, you can back off the charge when practicing. The G4 also has traditional rifle sights, or at least mine does. I like the idea that if I have some sort of optical failure, I can remove the scope in the field and still use it.