I’m creating new project, add some content from the market and they land in main ‘content’ folder. What I want to do is put all the imported stuff in to a sub folder but so far all my attempts to move them end up in either crashing the editor or in duplicated folder structure. any way around ?
The easiest way is to just select all folders in the content browser - move them into the new folder. Otherwise go to your project folder - content - select all folders - move them 
If you mean in file explorer then it breaks stuff, for example meshes lose assigned materials. if you mean moving in editor I get duplicated folder structure or a crash
Yes that is correct, it can sometimes break the referenes. To make sure nothing breaks, always move files/folders from the Content Browser inside UE4. Just drag and drop the files into the folder you want to move it to and select move from the popup.
Once moved you may need to manually delete the old folders.
seems that main problem is with the demo maps, they keep references to old file locations or sth