If you're going to apply, work on your English grammar. Look up subject-verb agreement.
Avoid killing yourself. Work hard, but don't break yourself.
Easy if you're not a tool. There are more grad students than undergrads here so you should have no problem finding friends...
Rent varies depending on what you can live with. I have a pretty nice place with 1 roommate for 1300/month each.
https://idss.mit.edu/wp-content/uploads/2017/10/PP-4-October-2017.pdf Seems there is no obligatory TA position from the department's perspective, but you can definitely hold one if you want. Also, if your project has sporadic funding, you will likely have to TA for a semester or 2 here and there.