I like the Rossi '92s being imported by EMF. They stay truer to the originals, and don't have the ugly bolt safety on top. There's no good way to scope one, although it'll work fine with a peep sight on the tang. The Marlin '94 is your best choice if you desire a scope on your lever gun. The Marlin is also easier to take apart for cleaning, and it's pretty affordable.
That being said, my latest lever gun for black powder is an EMF/Rossi Hartford in .44 mag. I ordered it from Steve Young, who also handles warrantee work for EMF. He slicked it up, and it's much smoother than factory original Rossi '92s I've owned. Although I shoot black powder for CAS, I plan to load my 44 mag cowboy guns with full power smokeless loads during the hunting season. My revolvers are stainless Ruger Vaqueros, so they make nice trail guns. The lack of adjustable sights keeps me from using them as primary hunting arms.
I think that a factory stock EMF/Rossi would work just fine for hunting, plinking, and most cowboy action shooting. They start at around $320 or $330 USD on gunbroker.com. For a bit more, Steve will sell you one already slicked up. Of course, shipping may be way too much for you to consider doing it that way.
As for the most flexible round, the .44 mag wins, although it's nearly tied with the .45 Colt, especially for experienced reloaders. The .44-40, due to case design, can't really approach the same performance as the other two rounds. Rabid fans of the .45 Colt and .44 WCF may pop in here and explain in great detail why they feel their pet rounds are better than the .44 mag.
777 leaves less fouling, but it still draws moisture, and it's still supposed to be cleaned up with water, so I don't bother with it. I've found Pyrodex to be worse than real black powder when it comes to causing corrosion. I'm ordering in a case of Swiss, since by all accounts it has higher energies, less fouling, and more consistent performance than other brands of black powder available in my area. The Goex that I shoot all the time cleans up pretty easily, and I've had no rusting problems, even though I live in a rain forest. Cleaning up after black has been easier than cleaning after some jacketed rounds and smokeless powder, and you don't really have to do it within just a couple of hours. Black powder takes a while for the salts to cause a problem, sometimes up to a couple of weeks, depending on where you're at.
Just to give myself a warm, fuzzy feeling, I ordered some of the Kleen Bore Inhibitor anti-corrosion gun storage bags. They emit a rust preventing vapor, draw moisture away from the gun, and they're supposed to last for up to 3 years. My guns all sit in these bags when not in use, on top of being in a safe with a golden rod dehumidifier.