1&2) Just insert the Ubuntu cd and go on with the installation. Select manual partitioning and use the same partition you used for Suse. It will be formatted and can be used for Ubuntu now.
3)XP installation will not be harmed in any way till you do not format that partition. Since you seem to have prior experience with partitioning, it won't be a problem.
4) It is better to use GRUB as your boot loader rather than ntldr. To make XP as default selection just make the following change in /boot/grub/menu.lst
Here x is the number. x= postition of windows entry in grub - 1.
Usually for ubuntu x will be 3.
5) Ubuntu will automatically mount other drives usually
6) GUI is enabled by default. Why do you want a root level access? Give me one good reason. Forget that Macro**** mentality of always using root/admin access. It is a big security risk. sudo can give you the required root level access.