Hi, the car looks great! Nice and standard is the way to go

The coolant system has loads of components and pipe work so any pipe or any component can leak there isn't really a main area that is common. However when coolant leaks it normally leaves a white residue especially when the coolant has leaked onto something hot like the engine.
Also if you top up the system and run the engine at idle, have a look as you may see the leak spraying out somewhere.
Look around the radiator/pipes,
the metal water pipe that runs along the right hand side of the block
expansion tank pipes and cap,
check your footwells aren't wet (could indicate the heater matrix has blown
and check under your car around the water pump and thermostat housing.
to get rid of the browness you will need to do a coolant flush and it sounds like you need new coolant anyway since its winter and you've lost a lot?
My grille sat the same too. However mine definately was in a shunt! and even with a new grille it wasn't right.