JA2 Gold is the original + the original expansion pack (Unfinished Business) and it’s the one you should get. Jagged Alliance 2: Wildfire is a much later expansion pack that you don’t need.
Weirdly, it looks like it might be cheaper to buy Wildfire and then you can get basic JA2 as a “dlc” for wildfire, but I haven’t tried that. Shit gets weird twenty years on, I guess. (don’t do this, see below)
I have JA2 Gold on Steam and it works fine. No idea why there aren’t any reviews. I had to buy it because my disc is scratched up. That’s right, I’m old enough that I have it on disc.
Regarding 1.13: 1.13 is absolutely a fanpatch made for people who have played the original, and while it adds or fixes a lot of things that were not in the release, it also makes an already extremely fiddly '90s game even fiddlier. If that’s your thing, maybe try it, but I wouldn’t say it’s necessary. If you’re coming at JA2 from NuXCOM or something, the basic JA2 experience is going to be super fiddly for you as it is. You don’t need to remember to drop your backpack when you get into combat and spend time comparing even more kinds of guns.
General advice for New People playing JA2: Your mercenaries will grow and become more skilled as they do things, so don’t worry that you can’t afford the really expensive guys at the start. You will want to have someone with a Mechanical skill, and ideally a doctor, and maybe someone with good Leadership for training militia. You will be given a Medic on like the second map of the game, but she’s not great and it doesn’t hurt to have two medics.
It can be worthwhile to bring in one of the REALLY high-end mercenaries on a 1-day contract at the beginning of the game because you will get to keep their gear (as long as you pay them for it when you hire them), and weirdly the first part of JA2 can be harder than the midgame because it is An Endless Quest To Find Rifles.
Sniping is very powerful in this game