Cross realm means that you can group with people who aren't on your server, although I'm not sure what the exact restrictions are in the EU. So staying on the server you are used to where you have your resources would probably be the smartest thing to do. To my knowledge none of the attempts to create an active boxing guild sticks for long, and it is usually most convenient to just have your own damn guild for sharing materials and repair gold across toons.

If you have a lot of gold you can use that to buy wow tokens for time, although the prices are not a great tradeoff and you may not want to spend your time farming half a million gold a month just to burn it on tokens.

All accounts are unlocked through WOD right now, I'd focus on lower level teams and let legion take care of itself when you are actually ready to play legion x5.

If you have already leveled a healer/tank combination you'll be surprised that leveling healer/tank/3dps is actually easier because you don't have to worry about other people doing stupid/annoying shit.