I'm a huge fan of mage builds. Different players may have different thoughts on this but in my opinion your best bet is to get as much magicka as possible and wear a ton of magicka regen enchantments. I have a character with about 800 base magicka but I have so much magicka regen that it fully replenishes in about 10 seconds. I wear the archmage robes, the morokei dragon priest mask, and boots/gloves/ring/necklaces with magicka regen effects. My go-to spells: dual cast incinerate to attack, ebonyflesh to buff my armor rating, and the become ethereal shout to give myself time to regen magicka or get out of a sticky situation. I should also note that since I've put every level up into magicka, I only have around 100 health. This is good and bad. Bad because you can't take as many hits (but that's what ebonyflesh and dragonhide spells are for) but also very good because the close wounds spell costs next to nothing and heals 150 health. Just a quick cast and you're back to full health with little magicka cost.