yep so NMEA is the governing body of marine electronics "in a way" and their 2000 protocol which is roughly based of the CANBUS protocol used in cars and a bajillion other industries, the major difference being the physical style connections etc.
So yep NMEA2K from brand A will more of then than not work with brand B "exceptions of cause"
though in some cases they make it annoying such as Garmin making their T connectors a different rotation to every other brand so when you join Garmin to Lowrance t connectors they do not line up.. Grrrr.
Lowrance make some of the cheaper kits. Maetron make some of the nicer and Furuno have some good hard wiring options.
A. Yes a lowrance NMEA2K starter kit will work with a simrad and mercury vessel view. happily they are all owned built or designed by Navico and work happily together.
The easier way to monitor fuel level is with a FLUID LEVEL SENSOR. such as the lowrance EP65R which when connected to the resistive sensor on your fuel tank will show what ever is remaining based on the Ohm output.
what sort of boat/engine do you have?