advice:

Dont get surly boat anchors (racks) if you want to ride centuries.

do you have a bike now? does it fit? can you extrapolate measurements from that?

as far as the steerer goes: buy it uncut, it will be ridiculously long.... Cut some off to leave it long enough to be bigger than youll ever really want, but not ridiculous. When youve ridden the bike for 3-6 months and know where it needs to be, cut it to the right length.