Don't know what the going rate for one is but I know that with the accessory hardware it's probably worth some money. Would it be more economical to sell it and buy a Pedersoli or one of the reproductions or a even a kit and build one.
Whatever you end up with I'd stick with a .45 cal. as bullets are easy to get if you don't cast your own. Go to about any gunshow or gunshop/sporting goods store and if they sell reloading supplies chances are they have 405gr. 45 bullets. 45/70's can also be backbored to shoot .45/90.
As to the strength of the action, the rolling block is actually stronger than the falling block. The rolling block was considered the strongest action in it's day. That is why all the long distance shooters (Creedmores, etc) chose it over the falling blocks.
If I had the bucks I'd love to get a Shiloh Sharps. You can get them in just about any of the old calibers you want but they start at about $1500 and go up from there plus there is supposed to be about a two year wait on them. I know a guy who has one that he uses in black powder cartridge silhouette and it's beautiful. I believe they have a website if you want to look. Sorry I don't have the url.