Here is my $.02 worth, typed quickly on my way out the door.

For the Garmin GNS 430, you would need the CDI for the VOR and localizer.

I believer that you need a CDI to certify any GPS for IFR. Let's say that
you were looking at a GNS 420 or 400 without the VOR. I think that you
would still need the CDI for IFR certification even though you can navigate
quite well without the CDI.

If you were not seeking IFR approval, then I think that you could get away
without a CDI.

The data fields on the map display can be reconfigured. I would suggest
configuring one to TKE (Track Angle Error), which is the difference between
TRACK and DTK (Desired Track). It is the amount of course correction that
you need to parallel the course. Configure another field to XTK (Cross
Track Error). These two data fields are enough to track a course. You have
two fields left, perhaps distance and next waypoint would be good choices.

The map provides a good reference and the digital data provides precision.
This is a very useful navigation interface and does not require a CDI to
navigate.

John Bell
www.cockpitgps.com