I've never been able to pinpoint an actual player level, but when you're at 100 enchanting you should be there, maybe a little over with potions.
If I remember correctly, destruction is head, body, ring, necklace. (some, alchemy included, use head, hands, ring, necklace....smithing is body, as is alteration)
Depending on the direction you want to go, you could say helmet/armor/ring/necklace for destruction and then add a magic resistance later. If you feel you're running short
on elemental resistance that way, you can get 25% from the Lord Stone and another 15% from doing the Mara quest in Riften. And whatever race you are, if you have in-born
resistance, that opens an equipment slot you don't have to use.