The general rule of thumb for a return pump is 3-5x the display tank volume after headloss. I stay on the higher side so 5 x 35 = 165 GPH. I would look for a pump that does 200-250 GPH at 4-5 feet of head. That is also dependent on the size of your sump, it needs to have sufficient volume to dissipate microbubbles and to contain any excess backflow in a power outage. Too small of a sump can lead to insufficient detention time for bubbles to go away even with baffles if the water velocity is too high.
It also depends on your overflow and what it will do. The photos you attached scare me. The overflow is not a very reliable design and the return is places too low in the tank so will drain down to that point when the power goes off and potentially flood your sump. The return should only be maybe 3/4" to 1" below the high water point in the display and the overflow should be behind a weir or in an internal overflow box or hang on back style and have a standpipe such as a Stockman, Durso, Hofer etc.
I agree on the Mag pumps, there are much better pumps that run cooler, quieter and draw less energy such as Eheim, Ocean Runner, Sicce, WaterBlaster or Tunze.
