I am a pure mage Breton and usually join the Dawnguards but i've heard being a Vampire Lord is extremely powerful if you're a mage and just wanted some help on deciding, I know how badly it scuppers you if you're fighting in the sun and some side effects are quite bad but i've also heard it can be extremely powerful if used right, it also fits in really well with my character an evil necromage corrupt by a lust for knowledge and Daedric artifacts but I prefer travelling in daylight etc. I just want to know if it's worth it

I guess the Volkihar is better for mages. Only the Ring of the Erudite is worth mentioning. It gives you 100 Magicka.

I would also grab the Necromage perk from the Restoration tree once you hit 70. It boost all your stats.