I can address the barrel length issue: Don't sweat it. I've hunted with traditional flint long rifles and never once felt handicapped by barrel length. And mind you I was prowling a lot of godawful thick stuff- laurel, greenbriars, pine slashings, etc. It's not my game anymore but I wouldn't hesitate to do it again- you learn to "snake" your muzzle ahead of you. It's a different mindset from adapting a short barreled centerfire carbine for such use (which I also happen to think is a pile of hogwash, but that's a topic for another campfire and another round of beers).
Also, don't fret the non-adjustable sights. These things aren't long range target rifles, and their trajectories aren't a handicap inside 100 yards. Regulate your sights for, say, 50 or 60 yards and figure out where the gun hits at 100 yards and hold over a RCH accordingly if you take a shot that far away. One thing I would caution you though is if your eyes are "old and sagging", have the rear sight mounted way forward of "traditional" spacing so you can focus on it easier. The reduced sight radius won't kill you.
Look at the sights on original Pennsylvania rifles and realize that those guys carved a civilization out of the wilderness with them, and if they could do it so can you. All it takes is practice and a willingness to do it.
Caliber? Not many, if any .54's to be found among original PA rifles, .40-.50 (and smaller) predominated. They killed game when it was a necessity for survival and managed it with those "smallbores" just fine- we can surely do it for leisurely purposes. I'm here to tell you a .45 round ball kills PA whitetails with alacrity, all out of proportion to what is intuited.
