Yes, simply replace the 125 with the GPS200.
Ted answered your question on the H2183. It's very well rated and actually the same unit Garmin private label and sell for more money. Anything Airmar (Maretron too) makes is usually very good. This unit has NMEA2000 & 0183 so pretty flexible in terms of what it can hook into. Having a compass added to your GPS will allow you to use radar overlay and I think also keep the boat pointing in the right direction on the plotter when it can't derived direction from the GPS COG feed. Most overlay software require 10hz output from the compass, which this has.
Kind of see that you are thinking NMEA2000 on all your interfacing. That is a good investment. Once the backbone is in place you just keep adding components and it all sorts it self out.....in theory.