Well, I apparently stirred up some feelings. Honestly OP if you go back and read your original post, it leaves the impression that you are simply looking for a basic head unit. I made a reasonable assumption that you didn't have a working head unit. Otherwise why would you say you don't care about nav and only care about basic radio and CD if you have a working stock unit? I was trying to save you from buying a $700, 10 year old nav unit only for CD and radio use when I know ECS has very low prices on new stock units.
Now I have installed a C5 A6 RNS-E in a B5 A4 before, so I can tell you that you will have to very likely cut a few mm from the bottom of the unit's face plate to get it to fit and there will be about a 2mm gap on the left and right side of the radio. The RNS-E was by far the more popular retrofit than the RNS-D. This was mainly due to the fact that NA maps were hard to come by. The biggest problem being that the disks had to be modified to work and that information was not widely shared. I see that the ad does not mention the navigation disks. Make sure you have a line on a copy of working modified disks if you ever want to use the nav functions. The good news is that the wiring a fairly direct fit for our cars if I remember correctly, though you'll have to verify this.
The main site for RNS-D retro-fits was a personal site that eventually evolved into navplus.us and then evolved into audiforum.us. Essentially this forum is what remains of the original RNS-D site and they may be best able to answer any specific questions going forward:
http://audiforum.us/cd-navigation-plus/
I also know that some units require the TMC module to communicate with the instrument cluster. I don't know what revisions these were off hand, but the unt was always pretty hard to find and there is no compatable TMC service here in the US or Canada so it is of no other use.
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