Full frame is overkill for a beginner. Lenses are much more important than bodies or format. You're much better off getting a second-hand APS-C body from a few generations back, and invest in lenses, than getting a top FF body and just a kit lens.
Depends on the budget of course. But you'll want to experiment and try different lenses, so that's what you need to budget for.
Which system/brand is the best - neither, it depends on your preferences.
First you should decide whether you want a DSLR (with an actual mirror) or an EVF camera. That's pretty important for the feel, and it's to your preference.
Second is the ergonomics of the system. Fujifilm and Olympus (or what's that they call themselves now) generally regarded as very "classical", Canon, Nikon and Pentax have their own styles that have evolved over 70 years, and Sony and Panasonic are more modern and more video-focused.
And then there are lenses. There's quite parity between systems so it doesn't matter that much which you choose, with the exception of Canon M system which has weak support and is dead. So the M50 is good only if you can get it cheap and treat it as a one-off for learning. (Ed: actually I think you can mount adapted APS-C DSLR lenses so there's still an upgrade path.)
I used to have Nikon DSLRs, but I'm not a huge fan of their new EVF offerings.
Sony is the biggest brand out there but I never liked their ergonomics and style.
Canon is kind of a gold standard, but always make weird choices re what can be in which model, and lately seem confused.
Pentax probably has the best ergonomics and controls ever, but is an extremely niche brand at this point.
OM system is lovely, but their smaller 4/3 sensor makes lenses different and is on life support.
If I was starting from scratch today, I'd probably go with the Fujifilm X system. Lots of various choices, been around for a while, good support, open lens system.
I suggest you look at Thom Hogan's sites bythom.com and sansmirror.com