Been there, done that. If there is not much sanding, a block and paper will do. If there is plenty, a small Makita with the appropriate grit is the fastest way to go. Then I would roll the paint on with a 3/8" nap roller Depending upon how thick the pickets are). Be careful to go over the face of the pickets to catch the paint runs.
As far as $, I would also go by the hour. What happens if some of the posts or pickets are rotten. You have to stop and repair them. If she wants a job price, make sure you CYA.
Hope this helps.