I voted 500 to 1000 UKP level.
If one buys the right machine to start off ... then most of that money would go simply for available parts. The right bike doesn't need an "expert" doing custom machining or engine hop ups or hand made Alu racks/panniers/tool box bash plates, Dakar Dash, $1000 GPS, steering dampers, et al.
Just the basics are OK.

But certain upgrades are nice to have when doing a longer ride ... but most of these upgrades all can be DIY.

With the right bike ... most all this "stuff" is for sale. Just bolt it on. Not much expertise required. As mentioned ... perspectives and needs will change once you "get out there" anyway. Make mods as required on the road, do the best you can. Have fun!

A RTW travel bike need not be a Dakar rally bike ... unless you're making a career out of ADV travel or intend to compete in a rally or be featured in films and magazines. Just know your bike's limitations (and your own) and plan accordingly ... and try to be as warm, joyous and giving as the poor people you may encounter.

IMHO, travel bikes are somewhat expendable. Use them up and move on. Don't get attached. Know when to park it or sell it. (cheap! ) But when on the road ... treat it like your lifeline ... it is.

Just a few things needed upgrading on my bike:
1. Slightly larger fuel tank (230 mile range suits me)
2. Better suspension
3. Better seat

Beyond this it's just a few simple DIY things: re-jet, hand guards, bash plate. Cheap soft bags, a dry bag.

The most important elements are done at the factory. The bike is either good or not.

If more riders learned to do good preventative maintenance and to take care the basics ... they'd get a lot further and have less trouble.
Bike travel is not for everyone, that much is for sure.