I bought regular 4 inch PVC and it has held up good. Here you can see both siphons side by side. I glued the joints with PVC cement after priming them, but also taped each joint with duct tape to make it stronger.
For my pond I needed 7 sections of 10 foot PVC, two caps, two 45 degree angles, and two tees. The total cost was just over $100.
You could use smaller pipe but it would drain water more slowly. It would be easier to handle and cheaper, though.
