priorities with hemi are 1. mana regen, 2. cooldown reduction, 3. AP 4. HP
boots of swiftness (can be fit in at any point while building deathfire)
zahonya (interchangable with rylai's if you are getting money fast enough)
last item optional, depending on the team, could be lich bane for some extra damage with regular attacks (and yes that is viable as with max cooldown it is very easy to spam turrets, rockets and grenades quickly) or it could be rod of ages or banshee viel if they have a lot of snares.
getting a ton of mana regen is mandatory, this lets you spam your spells constantly, hemi is a master at farming, harassing and defending when he does not need to worry about mana. cooldown reduction lets you put out more damage (important to have enough mana regen first to take advantage of the reduc). after that get AP to boost your damage and some health so you don't get instakilled if a lucky melee manages to reach you.
I've done excellent in my last several games, won 3 out of 4 with this build and in all of them i was far ahead of everyone else on gold and items and my kill/death/assists ratio were all very good. best one so far is 8/1/16