I have the same machine.
If you are buying Festool paper, Bob Marino sells 10 piece packages which is a great way to try something new. Granat is the general purpose line for sanding wood. Others are aimed at taking out old paint, buffing finishes, etc. Give Bob a call or send an email and he will school you and then some.
I've heard good things about 3M paper, too.
The paper is a little on the pricey side, but it lasts way longer than big box stuff.