I can tell you what I have done on our 54.
Originally our boat came with the Navnet3D system, two 502 Autopilots and the B&G H3000 set.
Although the Navnet 3D is not a bad system the power consumption is very high. Having tough about the situation for some time I decided to go for Furuno for most stuff as that was the most cost effective.
I replaced the course computers and control boxes with the new 702 generation. They work excellent. You may need to update the software on the computers if you have problems.
The NavNet 3D has been replaced with the Navnet TZ2 blackbox. This box can drive two displays independently so it now drives the inside screen and a new outside screen that replaced the 12" Navnet plotter that was there. I kept the radar, AIS and weatherfax as all work together with the new TZ2 blackbox. The instruments are replaced with Furuno's FI70. I bought a NMEA183 multiplexer and NMEA2000 bridge (Shipmodul Miniplex) to get data to and from the H3000 CPU and the NMEA2000 backbone. A TCP/IP bridge puts all navdata on the local WiFi network.
The system works very good and kept the costs acceptable. It takes about 25% of the power of the original system. But it was a lot of work installing it all. If you want I can send you the schematic diagram of the system.