I was also going to mention the Focusrite 2i2, which is all most people will need.
Small PA (thinking a powered mixer & passive speakers will work) - Could I use the monitor I use for recording?
Rather go passive mixer and powered speakers - easier upgrade path and many powered speakers can be kept and used as monitors when you upgrade your rig (or when you need more power for a bigger gig just hire a power amp and speakers and use your powered speakers as monitors.
Speaking upgrade paths - if you plan on growing and expanding, always bear your upgrade path in mind. Like the example above or buying powered speakers that have a matching powered sub which you can add on later as your needs grow. Make sure that when you buy your mixer, that it has enough features which allow you to grow without having to upgrade: effects sends; monitor sends; inserts; EQ and effects that can be switched to monitor sends; and
always buy a mixer with more channels than you need ?. This approach will save you a
ton of money over time.
Never a good idea to use studio monitors live for a number of reasons (not ruggedised for live use, different dispersion patterns, etc.
Mic (same mic I use for the recording?)
Yup - SM58s are the standard for stage. As your studio grows and you expand to better mics for studio purposes, it'll still be taking a pounding on stage (see: upgrade paths above ?).
Generally I try stay away from Behringer, not only for the oft-cited ethical reasons, but also simply because (IMHO) the more Behringer in a rig, the worse the sound. It used to be that they were the only real budget option, but there are so many brands now (and some of the better brands have produced downscaled offerings) that they have serious competition these days and spending a little more often gets you something far better. Also Behringer gear has very bad resale value, so it actually costs you more when it comes to upgrade time.
Obviously good,durable and quality cables
Learn to make your own and save tens of thousands in the long run. Use quality plugs and they will last you a lifetime, but start thinking of the cable themselves as expendables. ?
Whew! Hope that helps (it better, after all I've typed and thought about on this topic ?).