Dave, Without "seeing" the actual condition of your floors, it's hard to say how "good or bad the floor" really is. If the floor is in good shape (no deep scratches, waves, unevenness or rot/ware) and only the "finish" is bad, you may want to try and use a buffing machine.
You can rent one for about $25.00 p/d. You use this to "screen" the floor, which cuts/remove the layers of finish that is on the floor. An 80grit screen will probably do the job followed by a 100 grit. This is (In my opinion) better then a small sanding tool. You should vacuum between screening. You should also screen between each coat of finish you put down with a 180 or 220 grit screen and vacuum. You may notice that when you screen a water base poly that it has a tendency to corn/gum/ball up on the screen. Every 5mins or so, just clean the screen.
Just a note, you should start in the middle of the room with the buffer to get the feel of it. It's really not that hard to use, you just need to get accustom to it. Also, I think you will get a better result with Parks brand when Behr.