Brilliant, thanks a lot ddvlost! Tiling it is, then. Sounds more sensible indeed. Over on Reddit I heard “Engine handles stitching automatically, you don’t need to worry about that” so I guess that’s what you mean by “There is a tool that helps you import tiled landscape”? Will investigate.
Love to hear about the fully automagic streaming, can’t wait to see it in practice 
“All worlds that fit into world bounds (20x20km) should not use origin rebasing. I will disable it by default in next release, as most of the projects will now fit into new world bounds.” — I read in some threads that origin-rebasing is always advisable even in smaller-than-max-worlds for best physics/collision precision at all times, but not sure how dated that advice is? If it’ll be disabled by default in the future, seems safe to go with that for now.
Thanks again for giving me those helpful pointers/directions in such a concise and speedy manner, much appreciated indeed! UE rocks =)