The kind of wax you choose is 100% personal preference. Some like natural Carnuba; some prefer synthetic. The argument over which is better is similar to a Ford vs. Chevy debate: pretty pointless.
Hand wax vs. machine: Again - personal preference. Keep in mind that there's a big difference between a buffer and a polisher. Lots of people have buffers; few have polishers. You can't do much damage with a buffer unless you're using a really aggressive polishing compound and you're holding the buffer in one spot and applying a lot of pressure for a long time. (If you're using a buffer, I'm assuming you're not using a cutting pad.) I don't use a buffer around the decals, but I was ridiculed for that one. On the other hand, if you use a polisher - you better know what you're doing. The risk of damage can be much greater.
This is a great place to learn all about buffers, polishers, pads, etc. http://www.autogeek.net/
BTW, my comment about more than one coat of wax assumes you're using a natural wax. I don't know if the same holds true for a synthetic.