The components in the nav system are:

On-Board-Monitor - the screen and keyboard

Video Module - the TV tuner, only fitted if you have TV

Navigation computer: the device that takes the map discs, and calculates the route, and shows all the menus etc. It generates the video for the screen.

The dealer will be talking about replacing the navigation computer, which is the only device that can produce the problem you have. "Please wait" means it is trying to read the map disc and struggling. Either the map disc is horribly scratched and dirty (I am sure you will have checked this) or you are using a copied disc (harder to read). But your issue is highly likely a weak or dead laser, and replacing the laser will likely fix it. I estimate an 80% chance I am correct.
Dealers don't repair the nav computers, they only replace them in their entirety!
Lasers, or complete DVD mechanisms, can be purchased here:
http://www.navigatie-onderdelen.nl/website/index.php

Replacing a laser requires soldering skills and careful work.
Replacing the entire mechanisn is extremely easy, requiring only a torx driver and some long-nosed pliers.

If you want to buy a replacement Mk4, then any Mk4 will do. Newer the better, you don't want to buy another one with a weak laser :-)

When a replacement Mk4 is fitted (new or 2nd hand), it needs to be programmed to your car, my NavCoder software can do this, or you can go and pay a dealer (who will charge more than my software costs!)

Another tip: what software do you have loaded?
If your Mk4 does not yet have 4-1/100 (displays as 4-1/00) then upgrade the OS first, it may fix the problem.
You can display the OS version in the Settings screen, it's on the top right corner of the screen.

OS CDs are available on this website here.