Book of Thoth:
+100 Magical Power (Full Stack + Max Level = +160(+-15) Magical Power)
+125 Mana (Full Stacks = +875 Mana)
+15 MP5 = +15 Mana every 5 Seconds or // +3 Mana every Second.
Reason for buying so early is with your "Slipstream Skill" you can boost your speed plenty until you're ready to buy boots. At early levels the added speed of boots are not needed cause enemies generally are much more prone to run away. If you're engaged your spells give you a bonus to reduce their speed with "Zephyr" and "DOT THEM" = (Damage Over Time) with your "Whirlwind".
+50 Magical Power (Full Stack = +110 Magical Power)
+300 Health (Full Stack = +600 Health)
You can alternate buying this item first instead of "Book of Thoth" if you would prefer more health. Benefits to this are by the time you get 25 Stacks on this item you should be able to buy "Book of Thoth" thus allowing you to reach max stacks on both items at the same time. Downside to this is you don't get the early benefit of the +15 Mana which I've found to be more beneficial than the added health because you can dish out more attacks than the apposing team typically. Either way you need to buy both items first and build your stacks early on which will build your attack fast and hopefully if you play good will prevent the enemies from building any momentum.
Shoes of the Magi:
+40 Magical Power
+18% Movement Speed
+10 Magical Penetration
The "Shoes of the Magi" are an essential part of your build at this point, I've tested buying them earlier and later and being the third buy is by far the best. With a decent added amount of Magical Power + the Mana boost it adds an additional +7.5 Magical Damage if you take into account "Warlock's Sash's" 3% of Mana into Magical Power Conversion. The +18% Movement speed also allows you to escape most attacks if you time your "Slipstream" right. Also the +10 Magical Penetration allows you to basically void any armor that has been purchased at this point by the enemy.
+70 Magical Power
+33% Magical Penetration (= 13.33 Magical Penetration)
Buying this gives you very little but still quite noticeable magical penetration it will mean the difference between an enemy surviving and dieing. It also adds quite a nice amount of Magical Power. Not much more to say about this purchase except it gives you a nice punch for a quick buck you can rotate this purchase out after you have all buys for something that gives you more Magical Power or Penetration but I find myself to be quite successful without having to do so.
Rod of Tahuti:
+125 Magical Power
+15 MP5 = 15 Mana ever 5 Seconds
Passive = Increases Magical Power by 25%
At this point in the game it's safe to say if you're facing any tanks you need to bring them down, well if you're doing your skills right, have max stacks and are facing a fully loaded "Ymir Tank" then your "Whirlwind" will do about 120-140 Damage Per Second + Whatever else you hit him with + Zephyr which will deal about 550 and 850 if critical, you can really bring any enemy down. You also get a nice boost of Mana regeneration and should be around +25 Mana per 5 Seconds at this point.
+75 Magical Power
+25 MP5 = 15 Mana ever 5 Seconds
Passive = Ability Cooldown of 25%
The reason you save this for last is cause it's a good way to top yourself off at the high levels. You can crank out spells quickly, you have the mana built up and regeneration to make the cool-down effective. Additionally it adds a nice amount of Magical Power.
Based On Standard Buy:
Total Magical Power (Max Stacks + Max Level) = 838.2375
Total Health (Max Stacks + Max Level) = 2325
Total Mana (Max Stacks + Max Level) = 3078
Total MP5 (Max Stacks + Max Level) = 69 or 13.8 Mana Per Second