Welcome to Aquarium Advice, or AA as we call it, Stephen.
I'm not too much help with where to put bioballs, but I believe that they work best when water falls onto them, hence "wet/dry". so, if you set your skimmer to empty with a spray bar, or drip plate or something, you are in the good. I plan to have my return fill my skimmer intake and then to skim it as it comes out of the tank.
Skimmer brands: ETSS, Precision Marine, AquaC are supposedly creme de la creme. I'm 700 L is roughly 180 U.S. Gallons, so I don't think you could go wrong with a good AquaC in-sump skimmer, if you can swallow the cost. All good skimmers are not cheap. We'll see what the rest of the community has to say.
Once again, welcome to AA!! I'm still a newbie, but everyone here is so helpful!! Good luck to ya!