The NavNet 3D was the first generation Furuno MFD to use ethernet to connect to the radar. Probably you have a DRS12A or DRS25A? The successor DRS12AX and DRS25AX are still being sold. Your early unit should work with the XL.
You did not say which transducer you had. I assume you probably have a black box sounder now like a BBFF-1. You could connect the transducer to the XL directly instead and all you need is to adapt the cable connector to 12 pin.
The autopilot is pretty easy since it will either use NMEA 0183 or NMEA 2000 to communicate and you can manage that with an XL. It may require a NMEA 0183 to NMEA 2000 interface to get the heading data to the XL. You might want to gve serious consideration to adding an SCX-20 satellite compass. It is is proverbial "greatest thing since sliced bread" when it comes to accuracy of heading, course, speed, and position data. Your autopilot will steer better and your radar overlay will be a perfect fit.
I had a NavNet 3D system a long time ago. It was great for its time. A bit slow by today's standards.