I can answer your last question
They are very easy to install.

I have just fitted a Fridgoboat compressor with separate plate. Yes the plate and connecting pipes come filled with gas. Clever non-return valves in the couplings so no gas is lost when you connect it all up.
Just require power to the compressor, and a cable run from the thermostat control back to the compressor.
Only tricky part was running the copper pipes from the cooling plate to the compressor through the cockpit locker without kinking the pipe.
Compressor is air cooled and can be converted to water cooling later if required.