Standard AP Mid stuff:
Magic Pen Marks (these absolutely are not negotiable)
Flat Armor/HP per level Seals
there are really no good offensive seals, these are much more efficient, armor seals are fantastic in lane and health per level seals are good in teamfights. I prefer flat armor seals in general because they're more general. For champs other than Kat, flat armor is better, also you NEED them if you're going up against AD mid lane like Talon or Pantheon.
AP/AP per level/flat MR blues
I like flat MR due to winning trades lane phase (and I like being tanky). Anyway, any of those are fine.
That's your rundown.