What should I be using banked mana for in the early going?

Don't be afraid of upgrading the first couple of levels on your gear, they are not that expensive and could make your life a lot easier in getting to the gear that sells for more than you have earned so far combined.(guessing your are relative early in the game)

That being said I would recommend saving your mana until you can get mythical gear, you should be able to get to level 74 with out upgrading any gear by running Glitterhelm Caverns on hard or insane, and once you are level 74 you can farm some mythical gear from chests after first wave of Endless spire on nightmare even if you do not have very good gear. Then again don't be afraid to upgrade your gear the first couple of levels to increase your stats even if you will replace it soon, the combined cost of upgrading your mythical gear to level 10 is less than 1 million mana and you can get that rather easy once you can get mythical drops.

Mythical gear can be farmed with out any armor of notable quality, but the jump up to transcendent gear from mythical gear is huge so here you will have to have some high quality upgraded mythical gear before you can reach that.

For farming mythical gear I would recommend getting a Genie, you can find it in tavernkeepers shop sometimes or you can get it from reaching wave 15 of Hall of Court survival (doing it on normal should be enought for a starter genie but higher difficulty result in better Genie). The amount of mana gained from the Genie is relative to your weapon damage, so you should upgrade your weapon damage some to get more mana per hit even though your main damage source should be your towers at this point in the game.

This is all recommendations based on not going to shops and buying gear from other players. If you don't mind spending some time finding gear in player shops I would recommend saving up 10-20 million mana and buying a starting armor set(4 mythical armor piece of the same type for the 30% bonus) or 50-100 million mana and finding a core item for your set that you focus on upgrading first.