Need help adding Navigation to 2005 A6

[GJames97]

hello all,
My 2005 Audi A6 came with a navigation button on the console, but after pressing the button it reads "navigation not installed".

What do I need to do/buy to have navigation in my car at a reasonable price?

I've been thinking about two options.

1) What do I need to do (without paying dealer prices) to get navigation working through the MMI system?

2) If I buy an in dash aftermarker system, i.e. pioneer avic Z3 or avic f700bt would it fit in the dash and what model would you recommend? If I do this option, will the new system work as a new MMI system?

Thanks

George