Sounds awesome & it's a great cause. LHT's are versatile, lot's of components fit. I'd recommend having the headset & bottom bracket installed by a good LBS mechanic who can also adjust brakes & derailleurs if necessary. I ride a Disc Trucker w/559 mm/26" wheels. Some people claim 559/26" feels slower; to me no difference but I like the greater tire choice incl snow tires. (Heh, a Canadian friend once noted how American tourists visit in July & ask if it's going to snow.)

Basic choices seem entirely logical. I like drop bars esp for long distance plus aero clip-on bars. Aero bars can help while fighting headwind but perhaps more importantly can give a respite to back & shoulder muscles. But lots of hard-core tourists use butterfly bars or flat bars too.

18 gears is good: while 10-gear cassettes are working their way into the touring market, 9-gear still popular & probably more practical.

Schwalbe Marathon Plus a great tire; I rode Marathons on a previous touring bike & they were tough as nails, no flats. Heavy though & I'd guess that in wide-open stretches of your route that tire hazards would be low, so a lighter tire could save a lot of effort. Replacing a flat tube isn't fun but doesn't take too long usually.

I'd also suggest using a pedal that can work with both clips & street shoes. Sometimes bike shoes get uncomfortable after some time so it's nice to have that versatility.