As others have mentioned, you can use SPDs on a road bike. If being able to walk around easily in your cycling shoes is important to you, then SPDS are a decent option, as are Speedplay Frogs. I, personally, use Look Keo pedals. I like them, but there is a learning curve with them (as with most clipless pedals). I use SIDIs road shoes, but they aren't a good fit for everyone. I'd encourage you to go to your local bike shops and try on as many shoes as you can, both road and mountain. There are a number of brands in addition to SIDI, e.g,. Bontrager, Lake, Shimano, Specialized (this is what my husband wears and he loves them), Pearl Izumi, and Northwave. SIDIs are not known for their wide toe box, but they do make a "mega" version that works for some with wider feet. I'd find my shoes first and then choose a pedal system.
If you have bad knees, that might influence your decision. Some people with bad knees prefer a pedal that offers "float," i.e., lateral play that allows your knee to track with more freedom. Others will insist that float is bad for bad knees and that you're better off without it. If you take that approach, then it's imperative to get your cleats set up properly on your shoes. In this regard, Keo is a decent option in that you can buy the pedals and then buy cleats that either offer none, a little or a lot of float. I'd note that SPDs have no float, while Speedplay has unlimited float.
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