For MF purposes, I've never played a Hammerdin before so I'd have to go with a lightning sorceress. I've used such a build for my starting MFer on two occasions and she works great, even without the Infinity.
It comes down to balancing kill speed vs MF really. If you can do lvl 85 areas easily then running lower level areas really makes no sense. The only reason to run lower level areas is for some kind of 'other' pay-off. Eg. Boss, high density unique mobs, weakness to a certain element, high overall monster density, etc.
i wouldnt do anything less than 84. its just too much time invested for the payoff.
but yeah, places that are packed with unique monsters, or are crammed to the hilt with regular monsters, and are level 84/85, are in general, your best bet.