Can I be blunt and I do not intend this to be aimed at you O.P.
Like many, I use a lot of cycling forums and often we will get people coming in who have seen a blog, a video or a friend who has done it, about a world tour. I would say from that, 99% who are then motivated to consider it themselves, never do it even though they wish they could.
A better and more achievable way of doing things is try the U.K. first and for that most older mountain bikes with some decent considerations should be your starting point. I myself have done more miles on my old marin bear valley se with factory kit (replaced parts only when worn and then upgraded) than the very expensive new Thorn Sherpa I once bought.
General rule of thumb I have found works. The frame has no cracks, you have a new and solid bottom bracket and some semi decent wheels with probably schwalbe marathons on, racks that will hold more than you need and never take more than you need. Get it serviced so the brakes and gears work right, do some courses if you are unsure how to maintain a bike when there is no bike shop close.
If you need additions then a company like sjs has a huge variance of parts for you to customise
https://www.sjscycles.co.uk/
For most would be touring people the best time is starting small, learning your bike, things to adapt to you but most importantly, get out and ride, You are more likely to actually know what works for you when riding than typing on a keyboard. Remember touring is more than just the bike you ride but real world experience is priceless
In your particular case op, if the team marin is in good condition but you have no love for it then I would sell it and use the funds to build your build but to be honest, you can have as much fun on a couple of hundred pound second hand bike than a thousand pound brand new one. If you love the marin then simply adapt it
The link I posted shows just how many entry to mid level bikes can travel the world, let alone to tesco