Hello Billy,
The head tube length is 5". So your steerer tube needs to be at least 5" + the stack hieght of your headset, + thickness of your upper triple clamp mounts, if you are using a direct mount handlebar mount.
If you are using a stem that mounts to the steerer tube, as opposed to mounting directly on the triple clamps, as shown in this picture
Direct triple clamp mount:
then you will need the steerer tube to extend beyond the top of the triple clamps by a few inches, depending on the requirements of your stem. So if buying used, you would need to make sure to buy a fork with a steerer tube that hasn't been cut for a direct mount stem, or ideally one that hasn't been cut at all.
or the rear shock, to get 8.1" of rear travel you need a 3" stroke shock, to keep the geometry the same it needs to be 9.5" eye to eye. Eye diameter needs to be for an 8mm bolt. Width in between shock tabs is 1".
Of course anything will work, you can use something with less stroke, but it will lower the bike a little bit and have a little less travel. If the eye diameter uses a larger than 8mm bolt you can always drill it out, you probably won't find shocks with anything smaller than that.
You can find these cheaper sometimes if you keep your eye out
http://www.ebay.com/itm/2011-Fox-DHX-RC4-Boost-Valve-Shock-9-5-x-3-0-400-new-/230756276684?pt=Cycling_Parts_Accessories&hash=item35ba2559cc
Thanks-
David