For the pump size, it depends on your outflow. I got a commercial overflow filter. It had a gallon per hour rating.
I went with a pump with a rating of about 60%, and it was too much. Not too much to keep up, but too much water flow. I ended up with a 425 gph for my 55 gallon tank.
As to power outages, that can be an issue. My commercial overflow comes with a tiny pump the size of an aquarium air pump that connects to the top. In an outage, it will restart the siphon. I've had many outages over the years, most when I'm not home. I only once had less than a gallon on the floor.
I don't know what your drain system is going to be or how you might deal with power outages. They also make overflow switches you can put in the main tank to kill power to your system if the water level gets too high, but I always figured my turtle would bite or play with it, so I never used one.
You also want to use a drill to make a small hole above the water line in your return as a siphon break. If power goes out, you don't want your return to siphon all water back into the 35 gallon. Doing the return as a spray bar is even better.
EDIT: you might take some of these questions to a saltwater reef forum. Sumps are almost standard for them and there will be many more people answering than the one not very handy guy you get here.