Why do you need to sand? A paint scraper is typically all the prep I've done, especially on something that high above the ground.
You can spray outdoors, but then you're drastically increasing the amount of masking necessary. And doing it on a ladder doesn't work well, since you'll typically be too close to the wall surface for even coverage.
It really comes down to the width of the area. If you will need to move the ladder horizontally more than 3 times (~12 ft safe reach to each side, combined), scaffolding may be the better bet.
Not sure about scaffolding, since I've never used it myself, but extension ladders really need 2 people to safely move, especially once they are 25ft or longer. Otherwise you're asking for a window breaking or seriously jacking up your back wrestling with the damn thing.