That's nice, thanks! I should try getting a Frozen Heart on him as it provides that tankiness and a healthy dose of CDR. Do you have any other suggestions for tanky-CDR items?
Also runes. I suppose Magic Pen would be good on reds and quints considering that his Q deals big magical damage, but he also hits people a lot, so... is it more important to penetrate armor or magic resistance? For blues, either CDR or magic resistance per level seem like good options, but I'd like to hear what you think about them.
Spirit Visage is really great on him. Frozen Heart is indeed core, I'd say. For Reds, people either use MPen or flat AD. This depends on preference. AD is better than ArmPen for lasthitting, and your absolute priority in most matchups is FARMING and NOT DYING, not getting a kill (maybe after 6, but most of the damage will probably come from the jungler gank).
His ult also scales with AD, so you get both better lasthits, more damage together with the attackspeedbuff and a stronger ult out of it.
MPen is good for your passive and the Q and I like it better, but it really is surprisingly tricky to lasthit without the AD reds. Practice practice practice!
I personally prefer the MRes blues yeah. CDR is a bit overkill in my opinion as you will already itemize for it and you don't need the CDR that badly before you get your Ult / enough mana to spam Q.