Comboing: As long as you are hitting an enemy, the combo will go up. After a certain amount of time (5 seconds ish?) of not hitting an enemy, the combo will reset back to 0.
Super Armor: While you have super armor active, you can still be hit, but you will not get knocked out of attacks when you're hit by an enemy. Basically, lets say you have a 5 second cast for a spell. If you're hit, the spell cancels, you lose mp, and have to suffer through the cooldown. If you have super armor, when you get hit, your spell doesn't interrupt and it finishes. Hopefully that makes sense.
Builds: For the most part, you do whatever you want. My advice is find a skill simulator (there are plenty in the DFO section here at /gg), and pump points for what you want, then post the results in a thread, and have others critique it. Better to get it right the first time than mess up and have to fork out 25 bucks for a skill reset :U
PvP: It all really depends on a few things: what class did you pick (priest, fighter, etc), what subclass did you pick (2nd job, pretty much), and what skills you have. After that, its for the most part how skilled are you at PvP.
I hope this helps you out.
DFO: Astenhof 35 Soulbender, DoomPig 14 (going mechanic)