Hello, Stacy
My understanding of SDR 35 is that it's a gravity drainage pipe, not designed for pressure. It's usually buried in the ground, which helps support the gasket joints. The siphon system, while not building up pressure, does produce an extreme flow, and thrust, which might wreck havoc with the exposed gasket fittings of SDR 35. Glued joints are much more rigid.
You said, you found a good price on 12 inch schedule 40 PVC. Did you check the price of the fittings? It's astronomical!
From my understanding of siphons, the same size pipe used for a gravity drain will have a much higher flow rate when used in a siphon system. Thereby, allowing for the use of a smaller size pipe for a siphon. PVC pipe and fittings larger than 6 inch are really expensive.
Determine how much watershed your pond will have, and size your primary and secondary overflows accordingly.
