1) the text says it's for a 125, but the diagram shows an 85. Regardless, it would be helpful to know more about what's going to be in the final version of the tank as well as what other filtration other plan to have as well. E.g., if it's going to be a lightly stocked tank and you already have a HOB, this sump might work fine at 1.5-2x turnover. On the other hand, if it's heavily stocked with carnivorous fish and this is the sole filtration, more turnovers could be suggested. If you are going to use one pump and already have the UV, then what size is the UV and what are you trying to get from the UV (kill algae, bacteria, parasites?)
2) I think as long as you have 2 inches below the media, you will get adequate water flow. However, bio balls aren't very efficient as a submerged water media. I'd suggest something else like pond matrix or matrix if the media will be submerged like this.
3) the planted stock will not get a lot of deep water flow the way it looks. It looks like the entrance and exit to that compartment is simply an overflow at the top. Most of the water will simply flow along the top. If that's how it is supposed to work, then fine, but you might want more input from others that use plants in their sumps.
4) the baffles have to be wide enough to allow the water to flow in the desired directions without constricting the flow. You might want to wait until you have a better idea of the amount of flow through the sump. The faster it flows, the wider the baffles have to be.
