When we bought our Ericson 34, it had a mostly non-functional instrument set that looked a lot like yours. For us, that was a feature, as I wanted to have up to date instruments and everything wired to a NMEA2000 backbone.
To make the transition, I built a small plywood box to hold the old depth sounder display, and ran replacement wires from the old depth sounder transducer to the box. We stored the depth sounder display box in the lazarette and pulled out the box and put it on the cockpit floor when out sailing. Then I could take my time pulling out old wiring in the binnacle guard and mounting all the new hardware.
You likely won't need a discrete depth display; your B&G chart plotter will likely display it on the main display. On Teronadon's Garmin display above, there's a spot for displaying depth in the lower right corner. His display does not show a depth, likely because his depth transducer is not connected to the NMEA2000 bus.
I don’t have radar, but I do have AIS. Quite a while ago I bought a VHF receiver (ICOM M506) that received AIS signals and transmitted them on the NMEA2000 bus. My chart plotter now displays the AIS vessels as green triangles, or red triangles when there was a possibility of a collision. More than once I've seen that collision icon in front of me when a fast power boat is overtaking me from behind. All vessels over 65 feet need to transmit AIS signals, so I can see all the freighters, tankers, and ferry boats in NW Washington waters.
This example is of a Garmin chart plotter display and is from
here:
I found that displaying AIS vessels on the chart plotter was incredibly useful. I can see approaching traffic when it is still a ways off. So I later decided that I wanted to be seen on AIS, and purchased a Vesper Marine WatchMate XB-8000. Now I know that larger vessels can see me on AIS. If you decide that you want to transmit your location on AIS (like with the WatchMate), you won’t need a more expensive VHF radio that receives AIS signals. The WatchMate also receives AIS signals and puts them on the NMEA2000 network.
A
poll on this forum shows that 85% of the respondents had either AIS or AIS and radar. The comments in that thread discuss the merits of each.