You will need the BEP gauge.

You won't need to empty your tank. The sender will need to be programmed to the depth and capacity of the tank and the dealer should be able to do this. BEP will also program the unit for you. The units need to be placed carefully in the tank, not next to the sides or baffles etc. they are very accurate I did have one fail after a couple of years but the replacement lasted well.

Have you got an NMEA network with MFD in your boat? If so, definitely go the NMEA way.

The nmea based units, dont care what size your tank is, you tell it via the fish finder/chart plotter, how many liters the tank is, and when you fill it, or addfuel. It monitors usage and calculates remaining fuel, and range based on current conditions.(consumption versus speed)