Outside the spawn or in any other construction world.
You don't claim any land, you can just look for a place and start building there. It would make sense to mark out your construction area with banners.
Every Sunday ALL farm worlds are reset!
Yes, you can travel these at the spawn by the teleport table in the middle.
Farms above the Nether Ceiling are not allowed, generally any farm should be built server sparing. In the construction end there is a public Enderman farm directly behind the statue.