Regrettably most enemies seem to be immune against death, so Spell Focus:Necromancy seems not very useful.
Is Evocation a good choice?

Yeah, Evocation is a nice choice. It can help you in maximizing damage from the spells that inflict direct damage. Focus in Necromancy is not a bad idea as well. You'll meet a lot of enemies which don't have full resistance to death spells, especially if we're talking about the original campaign. In MotB there's more creatures with immunity.