Try an electric one. You could probably rent one from Home Depot. Man, I got a pneumatic DA sander and other tools, for what your wanting to do, at the house. I would just have to see it first hand to help you out. Other than renting at Home Depot, they can advise you as well. Just a thought.
Yes you can but if the paint/stain is latex/water-based, then the poly will have to be be also. You can't use oil based over top of water based. Minwax makes one called polycrylic that is water based. Another note- acrylic is water based and alkyd is oil based.