The bigger the better. I have a marina/finnex breeder box I modded as a fuge. Of course this is small for my 20 but the more water volume the better. I have a budget so I cannot be spending left and right. You could go with the CPR aquafuge if you are willing to spend the money.
My fuge is 1.5 gallons and contains live rock from my aquarium that I did not need in the display tank. I smashed it with a hammer to make it rubble. I have one compartment with Chemipure elite and chaeto with the LR. I also attached a clamp on light to help the chaeto grow. The ight i am using is a simple 6500k bulb from home depot. The clamp on light and bulb can be bought at home dept for less than $10.
My fuge serves as a mini copepod/nitrate controller. The chaeto acts as a phosphate remover and depletes the hair algae I once had of its phosphates in my display tank. The rubble rock helps house the copepods which can serve as a food source for my fish. My bio load is quite heavy for my tank. Predominantly lps and softies, I have 2 mollies, 1 yellow wrasse, a clownfish, and a firefish. Along with 20 blue legged hermits, about 60 ceriths, 4 or 5 nassarius, one peppermint shrimp, and an emerald crab. My tank is 20 gallons.
As far as a skimmer, I have a CPR bakpak. That skimmer holds a large volume of water. Plus I store two heaters in there (one for back up). I have a maxi jet 1200 running on it. just with that and the fuge I have running it has great water turnover. Plus my two koralias 2's.
The fuge is small but serves its purpose on my 20. You will want to go bigger if you have a bigger system. The aquaclear mod is practically the same as the fuge I have but the aquaclear costs anywhere from 50-$75. My breeder box fuge cost me $15 and the pump I took from a fountain (fountain pump:Beckett water pumps) which costs about $10 on amazon. Like I said, I am on a budget but this works great for me! Best of luck!