Begin by deciding what GUI (graphical user interface) you fancy. Lowrance, Garmin, Stadard Horizon etc. Then decide on chart supplier (Navionics, C-Map or Garmin) - their features and options vary. Consider that is Nice to have and what is Need to have for your use.
NMEA 2000 (and earlier NMEA183) are well established standards for communication between marine devices of all sorts. As long as compatible with within these standards you can easily combine makes and models.
That said my recommendations would at all times be to get the largest possible plotter display, running the software and charts of your preference. Personally I find Garmin too 'toy-ish' hence chose Navionics above it. Executed on a Lowrance HDS and combined with a Simrad (same company group as Lowrance) Auto pilot it makes an extremely rugged navigational package, combining all units' info. So the AP makes use of data collected by the GPS (like for deciding rudder activity against speed) and the GPS makes use of the AP fluxgate compass (in combination with the compass data calculated from GPS signals). The AP reads the Lowrance HDS sounder signal to use for steering by depth, keeping the boat away from too shallow waters.
All plug'n'play within NMEA, so pretty simple to install and connect. The operation of course takes a bit of manuals reading..
