tmackay -
Welcome.
First of all, read the post entitled "Black Powder Essentials" at the top of this forum. There are several posts in there with some excellent advice, including Part 1 of the series.
Second, with respect to "which gun to buy" - there is no single answer to that. A handgun is a personal choice, depending on many factors that cannot be easily described or measured. The only thing you can do is go to a gun shop and handle a few to see what your body likes.
That being said, it's likely you won't stop with just one. This is a bit of an addiction, so there's little chance of buying the "wrong" gun at any time.
There are some classic "starting out" guns: the .36 cal 1851 Colt Navy, the .44 cal 1858 Remington New Army and the .44 cal 1860 Colt Army. Frankly, you can't go wrong with any of those. They each have their pros and cons, and they are different from each other, but each is an excellent choice for it's own reasons. Over the past 30+ years I've found that the 1851 Navy seems to be the most popular, but it's a very small margin.
So go to a store and handle them. And if you can't do that, pick one, order it, and be prepared for some sore face muscles because you won't be able to stop grinning.
I'm not even going to try to weigh in on the HD issue. Any gun is better than no gun, given that you carefully follow ALL the safety rules.
And BTW, scrat, black powder, real or substitute is NOT corrosive. The combustion products are mildly corrosive, but the powder itself is not. That's a fact, not an opinion.