1. When I had a fridge-kegerator, I went with door mount.
2. Easier for the n00b I was. Never got in the way of kegs.
3. I went with nippes already on. In retrospect, I would have ponied up the extra cash and gone with tail piece. I'd have been able to clean the shank out and have more reusability. I just recently sold my nipple shanks off and replaced them with tailpiece.
4. Used a 5/8"(?) hole saw and a pencil.
5. Inside since I had a 5lb. Was a pain to mount so that the tops of the kegs fit right below the regulators and didn't get in the way. It actually rested against the temperature adjuster.
6. a. See the above.
b. One VERY IMPORTANT thing to remember is, if possible, seal the bottom of the your kegerator. I had a leak in one keg due to forgotten washer. Beer seeped underneath the rivets/washers in the bottom and some pretty nasty stuff grew in there. Worse, I didn't notice any of it(Due to it always being cold and upright) till moving day. I managed to clean it up, but it never work as well and eventuall died.
c. Get perlicks.
d. Get a used fridge. The worst part of part a. was that the shiner logo the SWMBO had painted on the freezer top door had to be taken off when I threw away the fridge. I wasn't weeping over the $100 I had spent on it and gotten use out of for 1.5 years.