Welcome back!
Starting with the simple last question first: All Auction Houses are linked region wide - so all servers on EU - but only for stackables, like herbs, ore, mats, pots, flasks. Individual pieces of armour are still confined to the server they are listed from.
A second piece of information that may be of interest is that levelling is insanely fast now. Current world record to get 10-60 is a bit over 3 hours https://www.reddit.com/r/wow/comments/yx53wj/i_leveled_from_1060_in_3_hours_and_wrote_a_guide/
If you are thinking of transferring characters, you should be aware that getting a new character up to 60 is maybe not as much work as you might expect.
OTOH, if you are transferring, do note that there are now discounts for transferring more characters.
If I were starting a new character, I would head straight to Silvermoon for Alliance, or Draenor for Horde. No hesitation, no doubts. Yes, there could be some lag during peak hours at the start of an expansion or, briefly, at a new patch, but for me, that is vastly outweighed by the world being alive. My main server is small (“medium”), but I also have characters on Silvermoon and Draenor, and the difference is insane, especially in this expansion where the devs are forcing us through garbage quasi-group content in the world that NEED large crowds to be viable.
If you really really don’t want to face the change of ever hitting a queue or lag, then I suppose Ravencrest would be an acceptable second choice.
Also, if you are getting into M+, and you want to avoid flamers and jerks, find a Community like Scared of Dungeons (Alliance) Scared of Dungeons EU EN
You will find more here: