Any level, but I suggest starting on one of the base game noob island and continue up the faction quest instead. It will speed things up more because Morrowind is a lot of running, overcrowded, and I *think* less skill points for the same amount of time invested. Can't confirm on the latter, but it sure feels that way because of the skill points I rack up from doing the faction quests compared to sticking to Morrowind main quests.
Server - PC/NA
Vhevet - (S) Night Blade/Female/Orsimer
Ecaer - (M) Templar/Female/Breton
Captain Beaster Bunny - (S) Warden/Male/Red Guard
Ezaera - (M) Sorcerer/Female/Altmer
Ecaeri - (M) Warden/Female/Argonian
Dun-and-Dunmer - (M) Dragon Knight/Male/Dunmer
What Can Go Wong - (S) Night Blade/Male/Bosmer
Izaer - (M) Templar/Male/Breton