If you have a 99.5 A4, then you should have a CAN-enabled instrument cluster. Does your cluster have a digital or analog clock? If digital clock, then you should be CAN"ready".
As for C-Navi vs. D-Navi, I would highly recommend the D-Navi as the map updates will be more frequent. As I recall, the latest C-navi mapsets just came out in 2004...and the previous update was 24+ months prior. Just my .02.
If it turns out that you don't have a CAN-enabled inst. cluster....and you really want to have the nav info in the cluster (which is highly recommended), then I would go with the C-Navi.
Either way, you have an interesting delimma ahead of you. Good luck!!