"Any pre-requisites required?"
Not really, the book is pretty straight forward. It helps to know a little bit about Linux and if you know the basics of the command line prompt and comfortable with it, you shouldn't have any problems.
"Can I keep my present distro if partioning my disk?"
You'll need a present working distro installed in order to install LFS.
"Does it require new material only or if I can do it with old (say P2) material?"
You shouldn't have any problems with that.
"How does it rate for difficulty, I have been working with Mandrake for 2 years.
Other then the Web Site and this Forum, where can I get additional help when needed?"
Difficulty, well if your just starting out with Linux and you don't know anything about it, it might be a little difficult to get going. Working with Mandrake for 2 years, and I assume you can work your way around a CLI, then you shouldn't have any problems.
More help, join the LFS mailing lists. That's probably the best resource for LFS. Then you can always come here, that's why we have its own dedicated forum.
"Is everything done at the CLI or if one can add GUI, such as Fluxbox or other GUI?"
Well, alot of it at the beginning is done at the command line, but no one is stopping you from doing it inside a terminal from your existing distro you have to start out with. Once you get the base and needed stuff installed then you can start by adding all the packages and programs to make it a usable system.
Hope that helps clarify things for you.