Nice work, seems like we are tackling a similar project at the same time. I bought the Tunze 8555 to fill the 5 gal RO reservoir in the sump area, and 3155 for ATO, but I really wanted to use the Apex stuff since I have it and will need to use it for the DOS AWC. I just keep hearing so much bad news about the Neptune ATK so I'm probably going to stick with the Tunze. What did you decide? BRS and my LFS people say Tunze 100% so it's hard to argue with that. See my build thread for more info.
As for cutting into your stand, I'm not a structural engineer (but I work with them), 4-5" would scare me, I suppose the force is straight down, so you'll have plenty of structure left, but I'd still be concerned about it buckling, maybe someone that actually knows could chime in.