Chaos magic. Is it necessary?

= a massive mana drain. Although the effect exceeds all expectations. =)
Perhaps, if you take Winraiel and max out his spell power, it might be worthwhile.

But did the elves complain about a lack of mana? I think it's the opposite - they have too much of it.
An enchanted Ballista is, of course, a rare thing (2% on Machines), but you can definitely use Rain. The total damage from Chaos with the new Druids is greater than that of a Warlock.
And, by the way, forget about Spell Power. What's the point of theorizing without knowing the details? It doesn't work with Arrow. It only works with a "pure" hero cast.