I am sure you will get some varying opinions, but here is mine.
1. I use KDE
2. you have tons of room. I have about 4G for linux, rest for win (which I use more)
3. you may not need to format, but will have to choose mount points. I use several partions, check the install info or linux sites for some strategies.
5. you can add it later. I had fun installing and running it between two computers, but if you are new to linux, probably enough to do without it.
just my two cents, sure some of the linux pro's here will have more advice.