Big hammers are nice to have. I think the first thing you have to determine what the "average" size of stock you work is, and purchase a hammer in line with that need.
Most people I know have a hammer in the range of 50lb, think about it, you just may have to take a heat or two more. I think control is more important, and if you do many different kinds of work, quick change type dies are a must. I just do mostly drawing and reducing with my hammer, so one set of combonation fuller and flat dies is all I need.
The other thing to consider is how much you are going to use it, a full time used hammer will eventually need parts, so you need someone to supply them also.