1. Yes, you do need to buy the HDD, as this contains the emulator in which you need to play the old games.
2. No attachment, you plug it straight into the controller.
3. Yes, you can use your old Xbox headset with your 360. You do not need to buy the attachment. As i said, plug straight into the controller. People have said that the original headset is better than the 360 one, but whatever.
4. The first batch had slight problems, but most of the major faults have been fixed so the ones in the shops now will be slightly better than they were when the 360 was first released. My 360 hasn't broke yet, and it was made in Dec of 2005.
5. No. It is not needed, as once again, i have said.
I know these Q's have been answered, but mine are easier to understand