Armor isn't really that important for a necro as stated, you are a caster so its really your utility and effectiveness killing mobs that matters. I personally haven't mained a necro so not so much help specifically on this class but each class has a handful of key usefulness items whether its stats, procs, or clickable effects. I would read the Necromancer page on the wiki and search for necromancer gearing or twinking guids. I'm sure there are plenty of them.
After a quick search on google I came accross this page
https://wiki.project1999.com/Equipping_a_Necromancer. Seems like a pretty useful refernce