I have the Garmin Ultra Plus 106 sv at the bow which is connected to Livescope on a Beatdown pole that is mounted on the right front side of the boat. I am considering adding a 7 inch Echomap unit to the console linked to the bow unit so the person in the back of the boat can see what the guy on the bow is seeing. Do I have to use an Ultra sv 73 unit or is there a less expensive earlier model that is compatible with the bow unit I have? Any other suggestions?

As long as its in the "Echomap" family, it will network. 73sv, 93sv, 106, 126, etc will all network.

Just know that the 73 and 93sv have much lower processing speeds than the Ultras. You'll find the mapping, switching views, etc to be quite a bit slower than the Ultra. I find the mapping most frustrating!