"1,800 is starting to get pricey. I could shell it out if convinced there is a big jump in quality and a much better long term investment."

Often people just get different stuff, not a lot better quality. Some touring bikes may spec disc brakes or STI, eithe of which are big dollar items, and no improvement in quality. May improve function depending on your point of view. Wouldn't be on my world touring bike though. More randomly, people just put different parts they like better and may end up costing more because of the volume in which they were purchased. Sound unfair? I think it does happen because a lot of the time the "custom" builder is really focused on only a few parts. They have a particular brake or saddle in mind, and the rest of the stuff becomes a mad search of online sources and the parts barrel. When building for market, I think the better brands are a little more careful than that.

It doesn't have to be that way. An LHT frame online with some substantial parts purchased at the same time, would be around discoutned to about
390. Between 390, and 1800, you have a budget of 1410. One really ought to be able to put together something better within that budget. That said, for world touring or comuting, rugged, less stolen, more easily replaced gear rules. It would be worth getting the mid range of Shimano simpler stuff (lx). Boutique stuff doesn't really fit in. I would buy a Brooks, some really good bars, And otherwise gut the sturdiest stuff. Like I bought a bunch of Ritchey headsets with roller bearings cheap but tough. Pick and choose solid stuff.

Or this:

http://www.brucegordoncycles.blogspot.com/