Cons first:
The modern miroku 1885 is complex to dissasemble in some places, I havent had the chance to handle the mechaum or C Sharps highwalls. Some versions of the winchester 1885 are pretty cheesy. I have no clue how easy it is to get replacement parts. The original 1885s are pretty expensive in an original configuration.
The 1874 is heavy, bulky, breaks the odd spring once in awhile (every spring in an 1874 is a leaf spring, the lever tension seems prone to breaking the most), if you dont put the hammer to half #### before dropping the lever you run the risk of shearing the firing pin tip off as the retraction is pretty weak. The full sized heavy barrel sharps with a 30 inch plus barrel is not something you want to carry around with you for more then an hour or so, then the nostalgia wears off pretty quick because you feel like your carrying around a road chisel. In any configuration, these guns are expensive, if your ok with 2 to 4 grand sitting in a quad scabbard, or getting touched with bloody hands or gloves or getting dropped or scratched up with tree limbs go ahead then. If they dont have what you want "on the rack" at Big Timber, get ready for a good long wait for them to finish your gun, a couple years.
Pros now:
Modern 1885 is pretty trouble free to use, its really a better gun then the sharps to use, you can get them in some pretty lightweight models if you dont mind the slap its results in. The modern ones are pretty cost effective for what you get.
Shiloh sharps 1874 is a better made gun by far, I shoot a 1874 Long range express and I wouldnt trade it for anything. When you do break a spring, give the factory a call and they'l send you a new one no questions asked. Its a pleasure to shoot. But I rarely hunt with it. The gun is pretty accurate when I do my part.
If its for strictly hunting, your better off getting a ruger #1, cheaper, stronger.
I good midway compromise would be a rolling block, again cheaper, and much simpler to assemble/dissasemble, its a buffalo gun as well. You dont need to worry about shearing a firing pin. A friend of mine shoots a pedersoli RB, and he outshoots me more then I care to think about. You can also have one built up on a swede rolling block action. Most of my hunting is with a 1897 rolling block in 7mm mauser, one day I'l try to make it more "buff gun" looking with an octagon barrel.
Any of the old style single shots are a labour of love to shoot and maintain, if you want to get set up for some target or gong shooting, add another 5 to 700 bucks for a good set of vernier sights, and another few hundred bucks to get set up for casting. If its strictly hunting, your good to go with the sights you have and store bought shells.