I wouldn't concern myself too much with trying to make a pilot talk to your instruments via seatalk. You would be $$ ahead if you bought a Raymarine EV150 pilot sans the pump. They don't come with a rudder feedback but you could probably use your existing one ,piggybacking the rudder transducer to the pilot and the ST60 display simultaneously.
Your RL plotter uses nmea 0183 and seatalk but the ex150 only communicates via nmea2000. If it were me, since it sounds like you're on a budget, I'd just use the pilot as a stand alone autopilot and be happy with it holding a heading. There are converters to change seatalk to nmea & vice versa but again, it seems like you are trying to save money.