I am interested in doing a play through as a mage that builds a party of adventurers and supports them. What are the best mods to greatly increase the number and level of enemies? I am looking for large scale combat and encounters that would make it so that you need a well outfitted party to be successful.

I think http://www.nexusmods.com/skyrim/mods/24913/? is the best creature-adding mod around. Most of the enemies fit right into the game nicely, in my opinion. And you can also configure the amount and also the strength of the new enemies in-game, via a menu.

Another mod you might want to take a lot at is http://www.nexusmods.com/skyrim/mods/3829/? This also allows you to tweak stats via menu. This one works on vanilla enemies.

Between the two, you should be able to tweak you game to suit your party strength with great precision.