The heart of your navigation system is going to be the display.
Before selecting a display you need to decide what functions you want to have available, both now and later as a possible upgrade.
Some users require a basic "Stand Alone System". This could be chartplotter only, or a combined chartplotter with built-in fish finder. These systems tend to be an "All in one box solution", and have little option of expanding the system at a later date.
If greater flexibility or functionality is required, then starting with a "Multi Function Display" is recommended. Multi Function Display's or MFD's can be used as single function devices or for any combination of the supported functions.
MFD's are generally capable of supporting chartplotting, radar, echosounder, side-scan survey, autopilot functions and entertainment system control,