Do you think that lugged frames really make a difference?
The 26" MTB seemed as the best buy from the start, but then I've read that lugged frames last longest.
Take a deep breath and slow down!

By my count you've got this thread running in at least 3 forums!
People tour on all kinds of bikes in all kinds of ways in all kinds of places. Have a look at this thread on CrazyGuyOnABike for inspiration:
https://www.crazyguyonabike.com/doc/?o=1mr&doc_id=8000&v=4jK
What kind of touring do you see yourself doing?
By that, I mean where? Will it involve lots of travel - planes, trains & automobiles?
Will it be on road or off-road?
Will these be long tours or short weekend trips?
Will you be self sustained, carrying camping gear, stove, food etc or more credit card style?
Are you handy with bike maintenance?
First of all, is there any reason you can't use your existing bike to get a taste for touring? I ask that because quite often we can have an image of what "Touring" is before we actually tour. After we've done a tour or two, our image can change, sometimes significantly.
It's very easy to get caught up in things like "I need the strongest bike" or "I need the lowest gearing". Apprehension, combined with some very effective marketing can convince us that we "need" far more than we actually do.
Again, I suggest that you try to get an idea of what and where you want to do and go and once satisfied, you'll have a better idea of what you actually need. And a great way of doing that is to see if your existing bike can get you out, actually touring, practising and learning. (Sorry, that's very boring!)
As for the MTB frame? I tour on one. It's my second touring bike. The first was wrong by pretty much all touring wisdom, yet I still managed about 12,000 km fully loaded on it.
This bike was put together based on what I learned while touring on the first. It's possible that the frame is not as strong as others out there, but my philosophy is this - if it ever breaks, I can pick up another frame for a pittance, transfer all my components over and continue. (Assuming of course it doesn't break while hurtling downhill!

) In the meantime, I have a go anywhere touring bike, equipped with the things I want (dynohub, usb charger, XL bottle cages, appropriate racks, strong touring wheels, great flexibility in tyre choice) for less than the price of an off-the-shelf touring bike without some of those features. It's a 7 speed triple and while I might have to look a little harder to find a cassette, they're available and cheap. Ditto with 8 speeds. I agree with
@MichaelW2 above.
As for buying second hand, I personally wouldn't be too worried about the components. I would however, pay a lot of attention to the frame to make sure that it is in good condition.
Good luck!