Plumbing is easy. What kinda room you got back there? The bigger, the better.
Small is 13.5", medium 19", and large 25". They're all 4" deep/thick (from back of tank to wall), and 12" tall. $89, $99, & $115 respectively at marinedepot.com. Get the light kit for it too.
The fuge has a pump that attaches to the intake and sits inside of the tank. The water flows back into the tank way of gravity. You won't be able to light the fuge opposite of the main with this setup, but it's still worth it to add water volume and a place for your pods
, macro algae, etc.
See the shot below. The pump will slip right onto the nearest balck tube you see. Then it'll overflow into the main back thru the pipe furthest away in the picture.
This a 25"er. HTH