A vintage, rigid fork MTB frame could work well for you, I agree. It's the cheapest path to a lugged frame for 26" wheels. Look for a high-end MTB if you want a light-ish frame and decent components; they can often be had for cheap if you keep an eye out. But, it won't be disc-compatible.

You won't find a vintage Japanese or Euro touring bike with 26" wheels, except maybe a '90s or later Koga Miyata.

Honestly, disc-compatible and lugged are at odds with each other; I can't think of a modern production frame with lugs and discs.
There are a couple disc touring bikes with a TIG-welded frame, but with a lugged fork crown:
Soma Saga Disc
Soma Wolverine
Surly Disc Trucker

And some non-lugged disc touring bikes:
Velo Orange Piolet
Surly Troll

Rivendell sells several lugged, non-disc bikes with 26" wheels (at least in the smaller frame sizes), but they are more expensive:
Clem Smith
Hunqapillar
Atlantis
Joe Appaloosa

In that price range, you could get a custom touring bike from many makers (Gunnar Grand Disc, for one example), but it wouldn't have the fancy lugs.