1) Sandbox, build what you want. Or survival, where you 1. Build a house to stay away from enemies 2. build tools to fight/gain resources. 2) Either pay at minecraft.net or get a crack (there's plenty) 3) I'm on linux right now. So yes. 4)Look at 2. 5) There are also servers if you haven't noticed yet... They allow multiplayer which can be quite fun.
it's very small,you can always get an up to date version from pirate-bay.just make sure it is one with the most seeds.(more seeds means more people downloaded it meaning it's safer)