Tips / Recommendations for building a long-distance touring bike on a budget?

Hey 'yall,

A good friend are planning to bike across Japan in April and May (we just bought our plane tickets, which are absurdly cheap from delta right now), but we're in serious need of gear upgrades. I've been training on a hybrid bike that's a little too short for me and not really rigged to carry all the gear we'll need to live on the road. That said, my budget to buy a replacement is pretty slim -- I can't just drop $1000-2000 on a new bike.

Today I went to local used bike co-op to poke around for a good steel frame, but didn't find anything that seemed appropriate. Maybe I don't know what I'm looking for or how to identify it.

I was wondering if this community had any good recommendations for budget touring bikes? There are a few UK bikes that fit the bill for me (the Flat White and the Galaxy AL both seem like good deals for starters), but, well, they aren't sold here.

What would you recommend? Is there a known complete budget bike that's well regarded for new tourists? Is finding a solid used touring bike and fixing it up the best bet? Let me know what you think!

(Also, general touring tips are nice? I'm a little afraid of wild camping and getting arrested, but I'm told it's easy and you get used to it pretty quick)