I'm pretty sure you have to buy an adapter for the Zephyrs to make them SPD compatible. The adapters will run you roughly $25.
I've had two pairs of SIDI road shoes, including the predecessor to the Zephyr. They're great shoes--if they fit. SIDIs have a narrow toe box, which doesn't work for everyone.
Is the deal you've found online? I would suggest trying some shoes on if you can at an LBS first. Be prepared to go up a size or two from your street shoes. You want them snug but not tight.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher