1. any backup plan is better than nothing. I use acronis home to keep a full backup on an ext drive. I copy my documents to a dvd every 3 months. I do not have a lot of new stuff these days.
2. as above acronis.
3. the biggest you can afford although at the moment I would go for 2 500GB rather than the new 1TB.
4. Yes you just have different folders on the drive for different pcs.
5. a very good practise.
6. I have c: drive for my progs and E: drive for my documents etc.