Brian, I have an electric DeWalt orbit sander, approx 5". My compressor is only 3.7 hp/60 gallon, 13 CFM and can't keep up with an air hog DA. I have a cheap $40 DA that is absolutely worthless to me because I never use it. I've read on here you really need a good one.
Anyways, my electric orbit sander is slower but it's good for someone like me so you don't take too much off to fast. I've found it works really well initially as a round flat board as you described. When I'm doing my initial 40 grit rough in, then 80 grit rough in it will practically circle, then highlight, then point an arrow to and plant a little flag in all the low spots. Needless to say I know where they are before I ever start blocking or guide coat. Then I move on to my Durablocks.
ALWAYS another tool you need!