Time Machine

I have switched my pc to a mac mini. I have an external drive with all my backup data. I was thinking of using Time Machine for backup.
1. Will it reformat drive to HFS and wipe out data.
2. When it backs up, does it backup only changed files.
3. Does it backup the whole drive including OS
4. If so I need an external as large as the macmini.

Thanks for the help

1) Yes, TM drives are HFS+
2) Yes, only files changed since last TM backup
3) Yes, unless you exclude folders manually
4) Yes, to be on the safe side.

Cheers ... McBie