"Best" is custom.. I guess the bike shop you do business with sells KHS & Specialized, right?

Trek's 520 is a good tool for touring, .. what Bruce Gordon builds is Better. His racks are a real highlight.

I assume you want 700c wheels?. since overhauling the Trek MTB, and converting it
is not being considered.

Surly LHT gets a lot of talk about ,here, made in 700c or 26"

Many Bike shops Have a QBP account, so can order those,
though may not keep in stock the Built Up bikes..
Ditto Salsa another QBP brand.

Tubus racks highly recommended, chromoly steel tubing..