1. The amount of levels depends on the console you are playing the game. PS2 for example has an extra level & Xbox has extra levels which can be downloaded. Standard amount of levels is around 10.
2. Yes, the games do vary from console to console. You can see the each games unique features here
3. There are 7 weapons in total, two of which are actual guns & the rest are various other projectiles. You can see all the weapons here
4. No, you cannot pick weapons that have been dropped by downed guards. This game is about you ability to stay hidden, running around firing a fully automatic AK-47 wouldn't exactly go unnoticed.
5. To a certain degree yes, no guard has to be killed, but some objectives require you to eliminate a certain indervidual.
6. On most missions you are not allowed to set off more than 3 alarms, but some missions are very covert and one alarm will result in mission failure. There are also a few missions which do not worry about alarms set off and so you have no limit, though constantly setting off alarms is not a great idea.
7. On most missions killing a civilian or bystander will result in mission failure.
8. If you spotted by a camera you will hear a voice saying that you have been spotted by surveillance and the alarm will be immediately triggered.