Sounds like a great plan, and a lot of fun.
This is a 58 cm LHT that I built up in 2010.
Easton seat post, FSA headset, Cane Creek SCX 5 cantilever brakes, Shimano Tiagra 4503 front derailleur, Shmano LX rear derailleur, Blackburn water bottle cages, 175 mm Sugino DX 500 mountain crankset (44/32/22), IRC square bottom bracket (68 x 103 mm to maintain 46 mm chainline with mtn cranks), Tiagra 4503 STI shifters (9 spd), Shimano XT 11-34 cassette, Sram 921 chain, Shimano XT hubs, Velocity Dyad 36 spoke rims, Wheelsmith DB spokes, Schwalbe Marathon 32 mm tires, Ritchey bars and stem, and SKS P45 fenders.
Frame was treated prior to starting build with Weigles Frame Saver, a rust inhibitor.
This is just the way I approached the build. There are dozens of other options that will work as good or better. This may give you some ideas, and might help you tailor the bike to your needs. Just go slowly and think things through. I have set up 3 bikes almost exactly the same way, and they have all been good performers.
The Sugino cranks are no longer available, but I believe a Shimano 442 square taper crankset with the 103 mm BB will work. I think they are still available at Chainline Cycles. If you go with a "trekking" crank, 48,36,28(26), the bottom bracket length will not be much of an issue. You just can't get as low of gearing as you can with a mtn. crank. Most new trekking cranks are 10 speed. However, if the trekking cranks have a 64/104 mm, 4 arm bolt pattern, it might be easy changing to mtn chainings.
It might also be good to get your bottom bracket faced and the threads chased at your local LBS. However, you it may or my not need it. You might also want to consider getting the headset parts pressed on at the LBS. Having the right tools makes it much easier.
A year later on a 3 month ride in Europe. Tubus "Cargo" rear rack, Tubus "Tarus" front rack, and Ortlieb bags all around.
PS. Park Tool's "Big Blue Book" is a good reference if you do not have much experience. Their website is also a good resource. There are a couple of places where left hand threads are found.