If you're just learning the game and it's your very first character or two, I'd suggest Toon Link and/or Doc just because they have the most versatility of your chosen characters. Bowser doesn't really teach you fundamentals, IMHO, because there's simply so much he cannot do, and he has a lot of unique character attributes (if you try to play Bowser's style vs stupid space animals, you'll probably get wrecked with almost any other character). His inability to really approach, setup an approach via projectile, awful dash dance game, and lag makes his style seem really stationary, or at least when I've seen good Bowsers play good people in tournament. Other characters benefit a lot from moving; you'll therefore learn more from moving because it's more generically applicable.
GAW I just think you'll get frustrated if your Fox opponents realize he's food for d-throw and there's almost nothing GAW can do about it because of his abysmal techs and light weight, and that disrespecting his aerials (shield grab and a lot of WD back, or running away so his fair weak-hits) is actually more effective than treating them like high-priority, decently ranged attacks with good anti-defense properties (what you want them to be). Also, shielding is very important and GAW's shield game is (MU dependent) either non-existent or mostly lightshield, which is significantly different than the fundamentals of shielding with most characters.
I also think that Doc and Toon Link having distinctly average hitboxes teaches you more about the importance of positioning, and how you have to aim non-disjointed moves (well, not ridiculously disjointed for some of Toon Link's moveset but you get the idea) to beat disjointed or other non-disjointed moves. I also think it puts the idea of stuffing and beating moves into perspective; GAW never really stuffs moves in the sense that the opposing move is supposed to win, but lost due to startup disparity. GAW usually just hits through things with his massive priority (if the opponent lets him).
I can do SHFFLs only with X fluently and with a lot of control and precision, but the buttons are comparably different (most people I know SHFFL with Y because they play Fox and it's closer to B, without going over A, so there's less chance of a misclick during double shines and SHDLs and other jump > B speed-oriented techniques).
Truthfully, SHFFL (and its components) and WD are some of the easier advanced techs to learn and apply simply because what they are is so obviously beneficial; nothing moves you back like WD so it fills movement niche. Similarly, walking takes a while to accelerate normally, but WD momentum lets you walk at max speed efficiently and moves forward a variable amount of space faster than walking most of the time, and with options you don't have out of a dash (some dashes are also slow). SHFFL's components just gives your air game versatility and speed. They're pretty easy to do; the harder advanced techniques to apply effectively are more mix-up oriented things like what to do at the edge, wavelanding off platforms, and various other miscellaneous stuff like pivot (which is IMO useless outside of pivot grab for the entire cast, almost, but whatever).
Sort of unrelated, sort of relevant; you don't know if the way you hold the controller now is going to be how you hold it forever anyway. For all you know, you're gonna claw or some other esoteric grip.
Anyway, I think you're at the point where thinking so much about everything in advanced is not beneficial. There's a lot of stuff to learn and apply. Just play the game for a while with your newfound powers with whatever character(s) you decide on, and if something feels wrong with a certain input style, either decide if it's worth sticking around and working through (decide if you think the hand method is the real problem or you're just not practiced enough yes and be honest with yourself), or modify your grip/input (I claw FCs, JC up smash, and ledgehop bairs because I feel I have more control so it's not outlandish!). Asking people in your local community to show you is also recommended; I'm sure they'd oblige.