Ok, I had to focus on this one!
There is a British guy, based in China, wrote an article on Crazy Guy on a Bike about his experience with custom building and how he ended up using a On-One frame - which he was very happy with. I’m too lazy to look it up.
Sifters really depend on how long you plan on touring. On my expedition build I use an XT 9 speed and it works well. If I was to do a multi-month tour I would probably switch to friction shifters and go 8-speed.
Trekking bars are great for touring, however I still prefer riser bar for trails and commuting.
Shimano makes good, inexpensive and widely available components and I would go with what you can afford and consider them replaceable. Shimano recently came out with a trekking group and it is based on the LX line and should be reliable but have yet to read any touring reviews about it.
I use OMM racks and I think they are great! They make heavy duty p-clips so you do not need V-brake bosses.
Also, I have toured on a lot of gravel roads, rail trails and singletrack and I prefer to use a rigid fork for the simplicity, reliability and the lightness.