As part of your answer...
How long do you think it would take you and your friends to complete the game?
Without knowing how long the game is, it is difficult to tell. Taking my experience with D:OS1&2 as a base, I doubt I will ever complete BG3 in coop. In both cases my coop partner got bored about 10ish hours and wanted to move to something new. Both D:OSs also get weaker from system and content perspective after initial act, so we will see if BG3 will suffer the same fate (considering that we don't get to titular Baldur's Gate city, I hope it won't be the case).
Still, even if game will be brilliant from start to finish we are talking about what is likely 100h+ campaign, which for the best experience will require all coop partners to play together at all times. So I might coordinate couple game sessions to hang out and catch up with old friends, but no, completion of the campaign seems very unlikely.
Would you want to play through again as a different class and/or race?
If I replay the game, than yes absolutely.
Whats the REPLAYABILITY FACTOR?
With variety of races/classes/choices and potential branching paths potental replaybility seems high. To me personally, it depends on game's quality more than exclusive per-playthrough content. I value depth and quality of the experience more than "playthrough exclusive" content. For example, for me to care about various character builds and party compositions, the game would need to be fine tuned and balanced to make playthrough with each build tactically satisfying and engaging. So far the game is too sloppy and too clunky to interact with to play it outside dropping each update to see what was improved. We will see how 1.0 will hold up.
Can you roll 3 wizards and build it and play it differently?
I think so, especially if you could "multiclassing" as still playing a wizard.
