If the top is completely flat, then it's a Flattop, if it has ears around the rear sight, it's not.
This is a Flattop.
This is not.
The 5240, 5241, 5242, and 5243 Lipsey's Convertible Blackhawks are Flattops, the 0308, 0318, 446, and 0463 standard Convertibles are not.
My personal advice is this:
1) You'll never save any money reloading for just a few pistol cartridges unless you're shooting incredibly high volume, as you might as a competitor. Talking thousands and thousands of rounds per year. The incremental savings on pistol and revolver ammo just isn't that much, when you only save a nickel per shot, it takes 10,000 shots to pay back even the most inexpensive entry level reloading kit. It takes around $500 to get set up to effectively reload even with a low end single stage or turret press, around $1,000 to get set up with a progressive. By the time most people get to 5,000 rounds on a single stage or turret, they replace it, or some other part of their kit, so their fixed cost per shot goes back up. For guys that start with a progressive, usually they've replaced some part of it, but their fixed cost was way higher to start with, so you still end up taking 10-20,000 rounds to pay that back. Most people will never fire 20,000 rounds in their lifetime.
2) You'll likely never save any money reloading, even at high volume. You'll find that you just shoot more, and either spend more - since you're replacing/upgrading kit too, or you'll break even and spend the same as you did before. That's been relatively universal for everyone that I've helped get started in reloading, nobody ever saves money in the long run.
3) Most Convertibles will never use one of their cylinders, especially for a reloader. If it were a 32-20/32h&r or 44mag/44-40 convertible, that isn't always true, because one of each of those is difficult to find, but for 357/9 and 45LC/ACP convertibles, both are readily found and there's not a lot of motivation to swap cylinders. The only guys that make that really work do their plinking with the 45acp, then hunt with 45colt, which can make sense to some, but not for most, simply because their hunting is not high volume shooting, so again, the one cylinder almost never gets used, even in that scenario.
4) Given your other firearms, unless you have a specific application for a single action, you'll likely find that you don't have use for it. Your other pieces are largely defensive weapons, which for most folks means they only really get used for plinking. Plinking with a single action takes a lot of time compared to a double action or semiauto, and it's easy for semiauto plinking guys to get bored loading a single action. If you're wanting to take up handgun hunting, then a 45 Blackhawk can make sense, but just for plinking, you might find yourself very bored, very quickly.
5) If you find that you do want a Blackhawk for plinking, and want to line up with your current ammunition, then I'd recommend simply selling off the other cylinder. You can recoup some cost that way, and make some other shooter that needs a cylinder happy. So then it's up to you to decide whether you'd rather plink with a 9mm or 45acp - the 45 makes more sense to me in such a large revolver, but it's more expensive than the 9mm. If I were going 9mm revolver, it'd be a mid-frame new vaquero, not a blackhawk.