Well like I said I run a emperor 400 on my main tank which suffices for pulling solids out of the water column in the main tank. At the outflow to your fuge you can place some egg crating and filter pad so that it runs through it before going into fuge.
in the fuge and your main tank sand be and sufficient lr
should suffice for your bio filtration.
There is a lot of debate when it comes to the bio balls. Some say they are nitrate producers over time. If you have enough LR
and a good live sand bed you could get by without them. I have never had them and Am and ites
stay at zero even when adding a fish or two to the main tank.
If it where me I would add a sump at the same time to house protein skimmer and any other mechanical filtration you feel is needed. Remember water runs down hill only. Depending on your overflow box your fuge will need to be 14 inches or so below your main tank surface level and if adding a sump it will have to be below your fuge level. Pump goes in sump and returns everything back to your main tank. In this case the water level in the fuge can always be to top of fuge tank. Sump level will have to be a little low to provide for back flow in case of power outage and a small hole somewhere above water level in main tank return plumping will have to be drilled to break siphon in case of power outage.
Topic of the month over at Saltwater.com is fuges and is worth a read. The author has hers set up as part of the display like mine and has lots of different life in it other than just macros.
No matter what you do you will most likely wish to make changes to it once you get the idea of how to go about it so go ahead and jump into it. Worst case scenario you have expanded water volume which in and of itself is a very good thing.