Basically what the title says I've made a female Imperial, her main skill will be archery, conjuration & illusion.
I know there's plenty of builds like this (where I got the idea from) but its my first like it so

I'm wondering if I should add one handed to my arsenal?

Remember you only have 81 perks. If you choose most of the smithing and enchanting along with your archery, conjuration and illusion, you are probably not going to have enough perks for one handed. Google and use the Skyrim perk calculator.

I am also interested in this build, but would probably opt for a minimalist approach. A beautiful, seductive Imperial under Dibellan influence. Maximum perks in speech and illusion (courage, calm, rally, fury, etc). Perhaps using some invisibility, poisoning.. but no weapons or destructive magic. Would try to direct a range of followers with diverse talents. Probably not very practical, but sounds fun to try from a rp perspective.

It's stupid to plan for level 81, it's way to high to achieve with normal gameplay unless you power level or exploit the game.

More sensible to plan to level 50, then just decide later if you get any extra perks.