headsets have distance specifications? or are you talking about the fork steerer?

I want the stem as low as it can go, to counteract the sit-up-and-beg geometry of the hybrid, so extension isn't necessary.

I've already measured the bike and found the fork to be a fit as near as I can tell. The fact that I must change the headset out is going to slightly alter the steer tube length, so I'm thinking I can't make a precise measurement.

At any rate, there is only ONE 700c suspension corrected fork that I can find online, the Dimension one.

So really, this thread is about the headset, something I know very little about -- what do I get?