There are reasons for staying with an older version of X-Plane. Maybe you like the way it worked better than the current version. Or it may be that the current version doesn't work on your system, because you're still running Windows 95 or haven't come to terms with Speech 5.
The FAQ in general have been updated to reflect the current X-Plane version. This page describes some of the differences in running version 5.54.
5.54 is long gone from the main X-Plane FTP site. You may still be able to find it through services like Download.com. I'm not aware of any X-Plane mirror sites that have 5.54. If you need a copy, ask in the Tech List. Someone can probably help.
Navaids, airports, terrain, and related stuff are in a folder called Additional Nav Data. Inside it, you'll find folders for
All the other stuff that's now in the Resources folder is in a folder called X-System Resources.