Rectifier to Step Down Buck Regulator
Looks fine, but I think you should have a much bigger mains filter capacitor (maybe 4,700uF or so). Also watch the ripple current rating on the capacitor. The one you have will have something like 6V ripple across it which is too much. 

Also watch that the DC resistance is not too high on the 20uH choke, as it's outside the feedback loop and will degrade the regulation. 

I don't think you need to do anything special to protect the input, the transformer will saturate. The basic LM2576 is okay with up to 45V in. 

Naturally switching regulators tend to take more computing horsepower to simulate than linear regulators. You can try LTspice and TI's web bench.