Mud X’s are mega grippy, they’re not quick or a decent volume though. Personally, I’d go for something a little bigger unless it’s an out and out mud tyre you want… The Race Kings look like a summer only tyre to be fair. I’m keen to give the Bontrager 29-3 tyres a go myself, the same as those that come fitted to the Gary Fisher Rumblefishes. They’re Tubeless ready too which is a bonus.
Fork wise, defo have the Reba’s set at 100mm. I found my 29er Inbred felt a bit twitchy with them at 80mm, as the head angle is pretty steep. To combat the front end height, I have a short steerer, an almost flat stem, and wide flat bars.
On the subject of bars, go for either flat or very minimal rise (like 1/2″ max)for definite, any higher and the front end will feel like its in the clouds…
Alfine rear hub, can’t go wrong in my opinion. Rim wise, the Flow’s are quite wide, in fact they’re very wide. I’d only bother with them if you want to run 2.4″ tyres all the time. Otherwise go for something narrower, like the Stan’s Arch, or the rather excellent Mavic TN719 as fitted to my Inbred.
Brake wise, well you’ve got to run a Shimano Centrelock disc on the hub anyway, why not run Shimano brakes? WAY more reliable than Hope and Avid in my experience. Buy some SLX or XT and have done with it… Superb brakes.
Good luck, and remember to post pics when you’ve got it built!