1) One was used by Coke, the other by Pepsi. They both function the same, they just have different disconnects that hook up to them (the plastic piece that connects the keg to the beer/gas line). They also have slightly different dimensions, but they're both 5 gallons. Personally, I prefer pin lock because they tend to run $10-$20 cheaper than ball lock right now.
2) Yes. They work soo much better than the cheap ones. If you can afford them, buy them.
3) For homebrew corny kegs, the only difference is ball lock vs pin lock.
4) Yes, but you save a lot of money assembling yourself. Example, Walmart sells one: http://www.walmart.com/ip/Igloo-6.0-Cubic-Foot-Kegerator-Beer-Dispenser-Refrigerator/15162475
5) I'd recommend picking up a cheap chest freezer on Craigslist. I got myself a 13 cuft for $50. I can fit 10 cornies in it. With a freezer, you just need a temperature controller to shut off the compressor once it gets to the desired temperature.
6) Again, check Craigslist. You can save a lot of money. I'd definitely recommend this route.