Leroux: First thing to know is that there isn't one set of D&D rules the games are based on. They are based on various versions of the pen and paper RPG (D&D 1st edition, AD&D 2nd edition, D&D 3 / 3.5, 5 ...) that can be significantly different from each other, e.g. in how they treat armor class (in the older versions lower is better and there's even negative numbers, in the newer versions it's the higher the better). So don't expect that playing one game will teach you all you need to know for playing another game.
EDIT:
I was searchng for a list of D&D games sorted by the editions they're based on, as I'm sure such lists exists, but so far I wasn't able to find one yet. :/
If I'm not mistaken, the Goldbox games (Forgotten Realms - The Archive - Collection 2) use 1st and 2nd edition, other ones from the first half of the 90's (Dark Sun etc.) and the Infinity Engine games use 2nd (except for Icewind Dale 2, which uses 3rd already). Neverwinter Nights uses 3rd and NWN 2 uses 3.5, as does The Temple of Elemental Evil, I think, and Sword Coast Legends is loosely based on 5th ...). The biggest differences are between 1st-2nd vs. 3-3.5 vs. 5 (I don't think there is a game based on 4).
As for the Infinity Engine games, Baldur's Gate let's you start with one character only, which seems less confusing but more restricted (you still get pre-made companions you can control along the way, but e.g. starting with a wizard puts you at a disadvantage in the beginning, especially when fighting on your own), while in Icewind Dale you get to create your own party of six, which could be overwhelming, but also gives you more insight in the different classes you can play. Icewind Dale is also a bit more focused on combat, which might help to learn the rules, or frustrate and bore you, depending on your preferences. Planescape Torment is really more about the story than about D&D rules.
Just to add to your great post, in Baldurs Gate you can create all six characters, start a LAN (can't remember exactly what the name is, possibly join game or something rather than just play) and click each of the empty 6 character panes to create 1-6 of the characters you want. Do remember though that if you create all six there is no room for in game characters to join, so you will miss the character specific quests in BG2.
Also, if you have the originals, use BGTrilogy or Tutu to combine BG1 into BG2 engine, makes play through smoother and you get the better engine (or you could use Big World mod which adds 30gb of patches, cut stuff, npcs, quests etc. to both games and combines them).
Icewind Dale is pretty much all combat, can't even remember a single thing about the "story".