I think, you have to put everything on the table for us, to analyse your gear ;-)
Seriously, changing the X100 for another M body, which you will use only for personal shooting and loosing one valuable digital body, that can be readily used for paid work?
The M6 is a grand camera - works with and without batteries and works perfectly well.
Getting a M2 or any other vintage Leica is a luxury, you don't sound like being able to afford (just for GAS ?).
I would look, to have my priorities on the gear, that earn the bills - if the 600 EUR valued X100 is doing that, keep it. If you see another piece of gear filling that hole better, go for that!
Honestly, I could see getting a good second hand Nikon D300/ D7000 or what ever as a much, much better option to switch your X100 into than any vintage Leica body, people will tell you, how great they are.
Do you have a backup body to your D700, that takes the same lenses (and preferably same batteries)? That's, where my money would be - you got the nice M6 and some glass for it already, as I understand.
Good luck with the big change - it sounds like an adventure of a lifetime!
Keep safe and take care (and keep us updated)!