It appears from the enquiries ive already made for my 172 whichever unit you go for you will lose the dashboard display for the radio etc although apparently you wont lose the temp and time display.
Sony units are the easiest to replace and apparently Kenwood do a few that will also fit however the leads etc are expensive. MP3 changers do not appear compatible as you will not be able to access the file folders although you will still be able to play mp3 files through, but you will not be able to select single files ( tracks). MP3 head units have same problems and only way to overcome this is to replace both the head unit and changer......
I think i got that wrong but im no expert and i HAVE had conflicting information from the 3 places ive recently been to to ask these questions so it is worth asking around until your happy with the outcome.
the only way to keep your display is to keep the existing head unit and fit a cd changer ( not MP3)
Ive been quoted around £200 supply and fit the Sony 6 or 10 changer.........
Good Luck