Welcome to the forum.
Whatever level you want to start at I guess. Most players would probably have some levelling done, but if you can find someone to play with from L1 then it’s possible.
Again, you need to find people who also only have the base game as multiplayer isn’t possible with people who have AoM installed.
No, you can use the same character in multiplayer and single player modes.
http://www.grimdawn.com/forums/showthread.php?p=619836#post619836