1. I wish I could tell you, but I just sent my Xbox 360 into Microsoft today. I had them all installed, and it was around 16-17 if I remember correctly, but let someone else answer more definitively.
2. I did about 4 sidequests (a pathetic amount) and finished the game in 34 hours. I also did very little extraneous leveling, so this is a pure run through.
3. Don't know, can't continue to play right now 
4. There are some related to key story events (about 250 gs for the lot), and and some related to beating the game on certain difficulties (Universe and Chaos I believe). There are also achievements for getting 10%, 20%.... 100% of battle trophies. Also for completeing quests, gathering data, and crafting stuff.
5. Battle trophies are a mixed bag. Some are easy to get, and a few are stupidly insanely difficult (like defeting a certain final bonus boss in a short amount of time).
6. Yes, there are couple levels of difficulty, and you have to complete the game to unlock progessively difficult modes. I've only played on normal and it had its difficult moments, but I can't attest for the harder modes. Also, I haven't played the last dungeon (Seraphic Gate thing), which of course is several hours long with no save points.
7. It is my first Star Ocean game, and I really enjoyed it. Characters are mostly enjoyable (keyword: mostly), but the voice acting was so-so.
"Your grasp of the obvious is inspiring." - Commander Shepard
"I'm Commander Shepard, and this is my favorite store on the Citadel." Commander Shepard
I support JRPGs on the Xbox 360.
Official member of the Xbox 360 Squad
"Nothing can make it moral to destroy the best. One can't be punished for being good. One can't be penalized for ability. If that is right, then we'd better start slaughtering one another, because there isn't any right at all in the world." - Ayn Rand, Atlas Shrugged
