I know ATO are more important for marine tanks, but I found adding one for freshwater helps at well. I have a 5 gallon jug/container that fits right next to my sump, and adds water to the return pump chamber. The benefit is that it keeps the water level essentially constant. I end up having to fill up the 5 gallon container every ~11days. I don't need to adjust my DC pump to keep the same level of flow. I have the SmartAqua, and considering the $$$ and time we spend in this hobby, I thought $150 was worth it in the long run. Just wanted to share for others. Of course, this is all moot if I had drip...but that's a different thread ;-).
Food for thought...if you plan on using PVC for plumbing, you'll also want to factor that in for head pressure (can't remember if anyone or I had mentioned). I have Herbie overflow on my 300 (including sump volume), but I only have (3) 1" pipes, including dry edrain. I don't have an over populated tank either though.
What you plan to keep aside, I think you will be more than fine with 1.5" drains, ball/gate valves and all.
I have looked at those auto top off resiovoirs and contemplated hooking one up. It's not a bad idea if I don't have a line run to it for now. Could you snap a picture so I could see yours?
Also on the returns I'm thinking about using hoses so I don't have to worry about the loss of pressures with 90degree elbows besides the one going into the back of the tank. And I think the 1.5" drains and gate should be fine but obviously we will have to wait and see if we are all correct!