my advice fwiw is to spend the next month or so reading tech articles & tips at autogeek, autopia, and properautocare. autopia forums have a whole section dedicated to machine polishing with a Porter Cable, refered to as PC. A buffer is probably never required since everything can always be done by hand, but a buffer will allow you to do a better job than you could do by hand and faster. Which is not to say a job done by hand isn't good enough.
Some products will specify for machine use only so don't use those by hand. Most all products that are done by hand can be done with an orbital. Normally you use an orbital for it's muscle to polish down clearcoat with an abrasive product but there are writeups on how to pop solid carnuba wax out of its jar, apply it to an orbital foam pad and apply it, then switch pads and remove the wax with a cotton or microfiber bonnet over a foam pad, to give you an idea. It's very versatile and imagination is the limit.
the preferred orbital, which everyone raves over, is the porter cable 7424, it has a 6" backing plate. Other model #'s are 7428 (7"), 7335 (5"), and 7336 (6"). I believe all are the same buffer machine, the difference are only in the size of the backing plate and the couterweights both which are accessories and can be easily bought and switched out. Not the cheapest at around $100-200 depending on the backing plate and if you get pads you get with it. But definitely one of the best in terms of quality and reliability. I've only read of 1 or 2 others, makita and maybe milwaukee, that have been used or can be used as auto detailing buffers (orbital not rotary) however I don't know how available pads and backing plates are for those. I think the foam pads, the good ones anyway, are all sized to fit the porter cable backing plate at 6 inches.
I picked up a 7336 with 3 or 4 foam pads from tropicare a while back on a group sale thru autopia for $151 shipped. With spring just around the corner, I'm sure they will be sales advertised over at autopia & autogeek from various sponsors on porter cable buffers, pads, and whole kits.
A good idea would be to plan out what you want to buy and get everything in one shot to save on shipping- if your not sure about claying still buy a jar of 1 or 2 bars for $20 because you will end up using it eventually. With polish either by hand or machine always use the finest or least abrasive to get the job done. If one isn't working, then step up to the next more aggressive polish, and so on. unfortunately you don't get the experience to know which to use or buy until after you spend the money to get a few different grades, try them, by which time your stuck with them.