Okay here's a piece of advice that might be an unpopular school of thought but is still the best i can give to anyone wanting to start with mugen or returning to it: Build your own collection from scratch... No seriously it might take a while but if you know where to look and what you want you can customize a build to your own liking and still know what you actually have in there.
Starter packs or bundles/mega collections are by now all VERY outdated and none of them will make it any easier to actually make your collection as you would still have to sift trough a ton of outdated chars and stages and thus spend more time doing that than you ever would building your own copy...
As for where/what to look out for you can actually just pop a question if any right here and someone will probably help you in due time...
lol, good piece of advice.
However, as the thread's devolved into basically just: "Where to find ____ character?" or "Gimme H-mugen game nao!", I've just ignored people's requests here.
Mugen was meant to be collaborative and customizable, and it's unique in that aspect. Unfortunately, people are so lazy (and sometimes understandably so), that they refuse to do any work to contribute, which is why H-mugen has died and pretty much every (good) H-mugen creator has either privatized their works, or hidden behind paywalls.
It's sad ... I've edited so many existing characters (One of my favorite things to do was add GoR CG scenes for more victims, which is a fairly straightforward if not tedious process), but I've gotten so disillusioned by the lack of support here. Hell, I used to be furious at guys like Mugenero for closing shop and just posting videos instead of sharing characters, but now I totally understand.
Also, word of warning that you have guys on MCM like that "C Restorer" who literally add like, a single frame or sprite to a character to "restore" it, and then repost to MCM. It's not bad, per se, but it's certainly misleading, because MCM has become more of a repository or warehouse rather than a new source of characters. Basically just MA again, but without ... certain people. And with that dude just spams reposts, except now he has a Patreon for doing nothing, lol. The only plus I see to MCM nowadays is that Moko_Moko actually joined and is pumping stuff out again.
And lastly, if anyone wants to start Mugen, Gettag's H-mugen is the best start (unless you can find the outdated QoF somewhere), and then supplement that build with stuff off of MCM.