By saying limit delete and restrict add/delete/change, I presume you mean forbid those actions for the user?
Note, folder can not be deleted unless all it's files are deleted first. So by forbidding deleting of files, folder can not be deleted as well.
To make file read only for everyone but the root, create file as root, set appropriate flags with
chmod 444 read_only_file.txt.
To prevent folder from deletion, even if it's empty, you need to make the parent folder read only. E.g.
chmod 555 parentFolder. This will prevent renaming, creating or deleting any files in the parent folder.
A bit simplier solution would be to always have one read only file in the protected folder, in that case there wouldn't be any need to set special rights for the parent folder.