A fighter class is a nice way to start off your mage. Kill what you can, and acquire loot. As you do, you will see that mage gear drops nicely if you can kill what was carrying it before hand...
Then, as you start to see a pattern forming, start piecing that gear together for your mage. It is possible to start as a mage and work you way all the way up, but there's just a lot of good gear out there that was once worn by a mage that is now in the hands of a monster.
So have it your way. As far as spells and progression in legendary levels, I use the default settings of Higher Ground Legendary Leveling system. I offer additional strength to casters with increased DC and spell damages along the way. I also have Legendary Rings of Power for casters, obtainable at levels 45, 50, 55 and 60. Once Demigod status is reached, more spells and power are to be had by all players, catered towards their classes.
Enjoy all, and as Despair takes you, you reach right into your gut and pull out what you need to give it right back !!!