#1 issue: the built-in trolling motor transducers for any brand/model are not SI capable, but you can use it for 2D only with those units. If you plan on using SI on the trolling motor now or in the future, the built-in transducer is a useless feature for Humminbird equipment.
#2 issue: the transom mount Compact SI transducer does not have internal shielding so you will need to exchange it for the XTM 9 SI 180 T transducer or upgrade to the XTM 9 HDSI 180 T transducer for use on the trolling motor for SI use with the 798.
#3 issue: if you get the 798ci HD SI, you can use ethernet to connect to the 898. If you can find the older model 798 then you will need Interlink to connect them together.