I;m gonna try a D20 Rifts game(don't tell Palladium! ME Begs You!) and I was wondering if the GM's guide was really necessary for any background, or if it's all listed in the basic book.
The main book covers the basics, which are then expanded upon with subsequent books. Sort of. The GMG is a complete collection of skills, weapons, etc. from all of the supplements to make things easier on the poor gamemaster trying to hunt something down, and contains basically nothing on the world itself.
You'll need the Rifts basic book, maybe sorcebook 1, and, depending on what kind of Rifts game you want to set up, one of the regional sorcebooks. The "Mega Sorcebooks" might turn out handy. If you have the choice, don't bother with the conversion book - it makes Palladium Fantasy stuff compatible with Rifts, and if you want that, just get Palladium Fantasy 2nd edition, and be done with.
Oh, and let me know what the finished version looks like - I might want a copy...