It all depends on what you are hooking up to it, how crazy you want to get, and how much money you are willing to spend. I have yet to take the plunge and rig up my shop but I have done a lot of research.
For table saws, jointers, planers, and miter saws you want a higher cfm rating and as large as possible sized ducting to handle all of these machines. You won't be able to get a high enough cfm rating along from a shop vac. For sanders or smaller tools you will want a good shop vac with a regular sized hose. The smaller hose and increased suction does better for fine dust.
As far as ducting goes, PVC is a cheap alternative to metal tubing. Ignore the warnings on the static causing an explosion, it is not a real thing. You will probably want to ground it though to avoid getting shocked every time you are near it. Blast gates are important too, make sure you map out your flow and have them at every T. If you want to get crazy with it you can buy automatic ones or make them yourself (saw a video on this of someone much smarter than me). If you are using a dust collector, start off with as big of a tube as your system will allow. Keep that main trunk going for as long as you can before it T's off without any bends. As you T off that's when you want to reduce down to smaller sizes for tubes (6" main duct down to 4" for a tool). Use rigid tubing as long as you can to the tool before switching to flexible hose. Because of the shape of flexible hose it creates turbulent air and causes cfm loss. Seal every joint with tape as well. Limit sharp corners, use two 45 bends where able to make a bigger sweeping curve.
Finally, again I have yet to do this myself but floor sweeps are handy. Put magnets on the inside though to catch any screws or metal that would damage your dust collector.
Yes I am an engineer and yes I have too much time on my hands.