beertech
As far as 1 or 2 transducers it depends on what you want to do. I have a transducer on the transom and trolling motor and use a switch to go between the two, you only need one for each boat.
I think that you are right that 898 has an external GPS, I would not let that stop me. The difference between a 7" and a 5" screen is significant. The large the screen the better for viewing.
You can get another receiver for around $100, yes it all adds up, you will need a power cord, transducer and do not forget to install the fuses. It is easier to make a full set up on both boats then just transfer the head. You can always wait to get a GPS receiver for the other boat or temporarily take the GPS between both boats.
George