I just skimmed the thread and was surprised that I didn't see Garmin's Helm iOS app mentioned. I think that adds another dimension to the OP's request for navigation electronics.
If the OP is leaning at all towards Garmin, I think a pretty reasonable, inexpensive approach is to put a networked Garmin MFD on the flybridge and make everything work from that. For the station below, you don't need to run video and remote control capabilities - that's where Garmin Helm comes in. It's a free app. You'll need an inexpensive router in the boat. But once you have that, the iPad you intend to have anyway is a full repeater for the Garmin above - all wireless over WiFi. Radar, sonar, instruments, charts, etc all come through on the iPad with the same touch controls.
For redundancy, the iPad can be a stand-alone chartplotter too with Garmin's BlueChart Mobile or a variety of other apps (having multiple apps is better).
As time goes on, it might make sense to add a real, second MFD below instead of using the iPad - that'll provide backup display for radar and instruments. Who knows. It might not make sense either. Either case, you'll have a backup.
I guess my message is that it's better to see how you'll use the boat than spending a whole lot extra up front on something you might find you don't really need at all.