First thing I would try is what I told you to try before use a lens clearner cd, and then also take the top off the drive and start up the xbox to see what is going on will give you more info about what is going on with the drive before going any further or spending alot more money then needed
as for them playing a dvd could be just a dirty lens or the laser read for a few and didnt finish reading and say play dvd , possilbly wrong dvd key.
Ever get any of your other xboxes fixed that you posted threads on?