Servo,
You can get a motorized valve for $300-ish, plus about 50-75 for the controls. I belive the Jandy electric pool valves are only $150-ish. You have to set them upo a bit different, control wise, but they work too.
An internal CSD would be a RCSD, using air to displace the water in the submerged container until it begins a "reverse siphon" and the water floods back in. I've built a few and they really move some water, but at the cost of a LOT of noise and salt spray. The spray can be contained and drained back, the noise is a bit harder to control.
I am currently building a tank with a RCSD built in. I'll post it someday when I finish it.