I have had a lot of experience with DA sanders. Do not buy Harbor Freight sanders. The Porter Cable sander is awesome!!
http://www.homedepot.com/p/Porter-Ca...46SP/202910698
I tried other sanders and thing would blow the others away. In the boat yard everyone (10 guys) all bought this sander after seeing it work. I could out sand them 2 to 1.
A Fess DA sander is also awesome, but really expensive. Fess is a 6 inch sander also. So you can use their sanding disks, since they have a wide range of disk grit options.
Your correct about knowing how much air a pneumatic sander needs. I made that mistake once. LOL