Do you want to InterLink another unit or connect it via an Ethernet connection. I would expect that you want to Ethernet connect another unit. Tell use what dollar amount you have to stay within, what display size and sonar capabilities you want it to have.
All 800, 900 and 1100 series units can be Ethernet connected. Those 700 series units that have the “HD” designation in the model name can also be Ethernet connected as well.