Howdy
+1 on MIke Venturino's book, Shooting Buffalo Rifles of the Old West. The best book out there to familiarize yourself with the old single shots and the old cartridges. Mike has a chapter on each design, Sharps, 1885 Winchester, Rolling Block, etc. He gives you the history of each model. And he has another section about each cartridge and how to load it. No Smokeless in this book, it is strictly about loading the old cartridges and shooting them with Black Powder. There are two more chapters, one about the basics of loading the old cartridges, and another on tips of the experts who win the prizes in BPCR. This book is really worth the money.
One thing to be aware of is that one rifle may not serve for both hunting and shooting at 300 yards. I have two rifles chambered for 45-70. An original Springfield Trapdoor, and a Pedersoli Sharps. My Sharps weighs close to 11 pounds. I would not like to have to carry it very far in the woods.
Regarding the other calibers, such as 45-110 or 45-120; I strongly suggest you start with 45-70 and forget about such more esoteric cartridges. 45-70 brass and other components are much easier to find, and much less expensive, than some of the more esoteric cartridges. And there is much more published data available about 45-70 than any of the others. 45-70 will put down any game that walks in North America. No real need for some of the other rounds. Mike agrees with me on this, if you are going to get a single shot, start with 45-70. After you have that under your belt, which may take forever, you can buy a rifle chambered for one of the other rounds. A friend has a tricked out Sharps replica chambered for 45-120. I want to tell you, that gun punishes you when it is fired.
Like most others, when I started looking for a Sharps I wanted all the extras. checkering, pewter forend cap, etc. But when I started handling rifles I discovered that I was not impressed with the execution of some of these features. So rather than buy a rifle that I was going to be aggravated about, I decided to buy a 'plain jane' model.
I found what I was looking for at Dixie Gunworks.
http://www.dixiegunworks.com/product_info.php?cPath=22_92_184&products_id=2576
Mine cost $800 a few years ago.
I also ordered a relatively inexpensive tang sight and upgraded the front sight too.
One other thing. Avoid crescent shaped buttplates with 45-7. My Sharps has a 'shotgun style' butt plate. Much less punishing than a crescent with a heavy recoiling round like 45-70.