Boyd, you are right on track. I agree that it is best to leave the cracks alone unless they are really bad. If you fill them when the wood is dry, like in a heated building in the winter, summer and humidity will expand the boards and mash the wood and the filler out.
I like the Minwax 2 part filler for most usage. If you have big parts of the floor that have come up when you removed the underlayment, you can kind of use this as a glue also. Depending on your particular situations, you can do things like coat the loose part with the filler and put it in place, put a piece of Seran wrap over the area and then put a heavy weight over this to act like a clamp to hold the part you are putting back in in position. Remember that you have to work fast with this though.
The drum sanders are the most common for rentals and do a good job. Since your floors are in pretty bad shape, you want to go this way. Remember that you will need an edge sander also and a scraper for the corners where you can't get a sander.