I picked up a revamped .50-70 rolling block last year and It's been a blast. If you start with a .45-70 Sharps you can use both smokeless and BP loads.
I hadn't reloaded with BP before, and it's quite different from standard smokeless reloading. I rigged a drop tube out of copper tube from home depot and some scrap wood. I use a volumetric powder measure and drop each charge down the tube into the primed shell. Then I use a compression plug to ensure uniform compression, stick an over-powder wad on top and seat the bullet. The goal is to have no air space at all and uniform compression.
You don't need to cast as there are a lot of bullets out there for sale, pre-lubed and everything. Check out Gunbroker too.
As far as gear, you'd likely need a compression plug in addition to the standard dies. Brass is the same as smokeless. Some folks use magnum primers with BPCR. Volumetric measure is nice, but again some folks weigh. I think consistency is the most important thing. Do everything the same way each time for each test batch.
Now when you get into advanced competition BPCR, things can get more complex and nit-picky. But you don't have to go that far to have fun. There are also alternate methods like using softer lead slugs and paper patches. But again you don't have to do that to start.
Here's a good book on the subject:
http://www.google.com/products/cata...a=X&ei=br14T6OxK8LhiAKatb2nDg&ved=0CG8Q8wIwBA
The big expense is often the rifle itself. A high end Sharps replica can run serious $$$. The actual antiques are ironically sometimes cheaper than the modern repros. .50-70 rollers like mine are not very expensive. .45-70 trapdoors are also pretty easy to find for reasonable prices. But you have to be pretty sure of the rifle's condition and be very careful about your loads. 145 years is a long time to be shooting.
If you're willing to fudge a bit on the historical accuracy, one option is to get a Ruger No. 1 in .45-70 and just use it to learn both smokeless and BP loading. It's a tank in strength, you would be hard pressed to hurt it. And it's about half or less what a Shiloh Sharps would run you.