I have decided to scrap my 4-5hour lvl 7 Nord warrior and go with a Khajiit assassin/thief. I figure I will use a good mix of my bow for ranged attacks and daggers for surprise attacks upclose. The one new part of my character that I have never really used in TES before is magic and alchemy.

How hard is to to learn how to use illusion spells and productive alchemy?

Not sure, even all the vets out there wouldn't be able to tell you, as this is not what they've played. If someone's used alchemy and illusion in Skyrim already and has gotten a feel for it, they will be able to tell you.

Alchemy is ... different ... than prior games. If you are going to use it at all, I'd say start right away. I waited and I'm regretting it now. First potion I finally tried making wound up being a combo worth 242 gold!

You will want to eat one of every single ingredient you grab because this tells you the first effect (of four). It's far better than wasting ingredients on failed attempts at making potions, and it will allow you to create needed potions like Restore Health and Restore Magicka right away.

And you WILL fail plenty of times, as with Morrowind, so don't get too annoyed.

You'll have to decide if you are going to use a lot of poisons (i.e., you're using weapons for combat) or you're using spells and buffs (i.e., you're pure or nearly pure mage). Of course, poisons may go with stealth if you are the assassin type.