Rooting is optional
Getting root or rooting your phone is the process of modifying the operating system that shipped with your device to grant you complete control over it.
Root only if you wish to :
- flash kernels, custom roms
- installed apps such as adaway to remove those pesky ads from popping out
- installed apps such as Titanium to backup and restore all your apps/data
- installed mods such as improving audio quality, screen display quality, etc
- installed tweaks to kernel, tweaks to undervolt/underclock

Rooting does not change any of your data nor will it delete any of your data. It is a very safe process
You can still use 64GB sdcard but must be in fat32 format. Root it to use it in other format eg its default exfat format

Rooting , read this self explanatory guide. If you still dont understand ask here or at XDA
You can select root with or without unlocking bootloader. Unlock bootloaders allow you more options in rom and kernel flashing

http://forum.xda-developers.com/showthread.php?t=2186368