The talk of length for such kind of game is weird.
yep, it's not the kind of game you "beat" and then you are done with it. Just like civilisation, every time you play it will be a different game.
I never really played the original series but really liked the demo and played 200+ hours of Jagged Alliance 2 and Incubation. Which difficulty level should I choose?
it depends how much tolerance you have for having to start over because you just got wiped.
Normal seems to be doable but not a cakewalk, while classic is when the game will actively try to kill you.