Originally Posted by TenGrainBread

If you are getting a groupset, you will have to do a drivetrain-only groupset as modern mountain/touring groupsets are oriented toward disc brakes, and presumably your vintage 26" wheeled bike will need rim brakes. Shimano groupsets are available cheaply on sites like Chain Reaction, Wiggle, Merlin, etc...

If you want bar end shifters-
I would look at either a Deore or Deore XT groupset. Deore if you want a triple crank and XT if you want a 1x. Just make sure the bar ends you get match your gearing, or that you run in friction.

If you want brifters-
Look at a SRAM groupset - GX or Rival or Force. They come in both a double and 1x option.

Regarding triple vs. 1x-
The nice thing about the triple is you will preserve your high gears (road-oriented) while still getting low mountain gears. The nice thing about a 1x is it's lighter and simpler (no FD or front shifter), which is good for off-road touring especially. You can get a good gear range with 1x if you get a wide-range cassette, like a 10-40 tooth. If you were privileging the Baja Divide aspect of this, I would go 1x. If the road tour aspect, I would think about a triple, but a 1x with a 34-40 tooth chainring and a big cassette would be suitable as well.

Regarding wheels and tubeless-
The only prebuilt 26" tubeless ready wheels you are going to find are meant for modern downhill mountain bikes. Which use disc brakes and wider tires than a vintage frame would accommodate. So if you want rim brake tubeless-ready 26" wheels you are going to have to either build wheels yourself or get your LBS to build them for you. Velocity Cliffhanger rims come in 26", rim brake, and tubeless ready and are made in the US. Those would be my recommendation. You will also need a tire that is explicitly tubeless compatible and there are plenty of 26" tubeless tires out there to choose from.