Garthine, as far as metal finishing goes, you'll want to either blue or brown the metal. Well, let me back up. Some kits you can buy, the metal is already blued, all you have to do is finish the wood and put it together. Other kits, the metal is still "in the white", or not finished. It's a matter of filing/sanding/polishing the metal, then bluing or browning it. If you get a gun with an "old time" look to it, you'll want to brown it, as that's the way they were way back when. Birchwood Casey makes a "plum brown" solution, that requires heating the metal, and applying the browning solution. Of course, they also make all the cold bluing type solutions too. There's lots and lots of ways you could do it. Brownell's carries a ton of products to do this. With your background, I don't think you'd have a problem finishing the metal if you wanted.
As far as twist goes, well, the deal is, first you need to decide what kind of projectile you want to shoot. If you want to shoot just traditional round balls with patches, you really really want to stay with a "slow" twist barrel, say 1:60, 1:72, or so. Maybe you could get by with a 1:48, which is kind of a medium twist. Round ball is traditional, and for many "purists", that's all they'll shoot. For hunting deer sized game, you'd probably want at least a .50 caliber. .54 caliber is a good choice also.
If you want to shoot a "conical" bullet, you'll want a faster twist, say 1:32, 1:28, or so. Conicals are much heavier than round ball, and elongated. The faster twist stabilizes them better, making it accurate. If you try to shoot a conical out of a slow twist barrel, chances are your accuracy will be terrible, and the bullet will be tumbling. Most modern inline muzzleloaders are set up to shoot conicals. For traditional guns, you can usually get either a fast twist or a slow twist barrel. Many folks like conicals for hunting because they are heavier, and transmit more muzzle energy. For deer sized game, I'd say .45 cal is probably minimum, with a .50 preferable.
Lots of folks go with a 1:48 twist, because it's a "medium" twist, and allows them to shoot either patched round balls, or a conical type bullet. But every gun is different, and sometimes a 1:48 barrel will shoot one type or the other much much better (or worse).
Sure, you can get a .58 cal, or even a .62 or .68. The "range" isn't any better with a larger caliber necessarily. You've got to use a lot more powder to push a .58 cal ball to the same velocity as a .50 cal. By playing with your loads, you can find out what charge your gun likes.
There's a lot of variables in muzzleloading. It's not like cartridge guns at all. But once you shoot a big bore muzzleloader, with the sparks and smoke flying, you will be hooked.
If you want to learn more, go check this board out, the guys on there have forgotten more than I'll ever know about muzzleloaders. They even have a section called "builder's bench" that deals strictly with building your own muzzleloader:
http://www.muzzleloadingforum.com/ubbthreads/ubbthreads.php