A battery monitor tells you state of charge properly, by counting amps in and out. Voltage alone does not, and guessing from voltage is how people end up with a flat bank and a warm fridge.

300A is the right size if you are not running a large inverter. Fridge, lighting, water pump, charging and solar all fit inside it comfortably. If you are running a 2000VA inverter or bigger, step up to the 500A or 1000A - a shunt that cannot read the peak is not reading your system.

IP65 sealed, so it can live somewhere that gets damp and dusty rather than needing a dry cabinet.

Specifications
300A shunt, 50mV
Bluetooth, VictronConnect app
IP65 sealed
Manufacturer part number: SHU065130050