I am not sure about the one post limit, but to block attachment and thumbnails to unregistered users go to:
Admin CP > Usergroups > Usergroup Manager > Unregistered / Not Logged In (Click Edit Usergroup), then under "Forum Viewing Permissions" set "Can Download Attachments" and "Can See Thumbnails" to No.
As for the links this mod will do what you need:
https://vborg.vbsupport.ru/showthrea...ks+From+Guests