I haven't played D2 in decades and thought to try it again. I forgotten how these runs work. Is there a min. Level our character have to be to join?
Are runs mainly done on hell difficulty?

Trist, tombs, and baal runs are done in norm.

Nothing is done in nm except my baal runs :) (to pad games while collecting spirit runes!)

Chaos, Baal, and Chaos+Baal runs are done in Hell.

Levels 1-15 = Trist
Levels 15-24 = Tombs (can switch to cows at 20, join a chant room)
25-40 = Baal
Get a grush at 40 (you are a bumper, people will love you)
---NOTE: Do not enter nightmare unless you are getting rushed. You WILL get stuck. There are no games in nm.
You can join Hell Chaos runs at any time for sweet exp.
Lv 60+ = Chaos + Baal. You can switch to just Baal later on if you want to mf during each run (I do this and occasionally get some sweet loot).