Hi, Erin. There are many posts on this subject already on the board. Start by skimming down the topic list and also check the FAQ page. We're happy to help but this general subject comes up often.
In answer to your specifics: 1. When you sand with a rented power sander, you should be down to where the floor is light and without stains. There are instructions here on how to handle some other stain problems. As far as what color to stain your floor, I would go toward a light colored floor. It will make your room seem larger and save you a lot of problems in the future if you decide that you really wanted a lighter color. Don't use a high gloss varnish finish. Killing the shine a little will make maintenance easier for you also.
2. You probably will have to scrape the glue. The old glue will clog up the expensive sanding belts that you will buy when you rent the sander. You can get a wide paint scraper and this should go quickly and be cheaper that using a stripper.
Be sure to get a good dust mask. You will generate plenty of dust. And be sure to seal the rooms off while you are sanding in them, otherwise the dust will go thru the whole house.
Good luck and come back with specific questions. Henry in MI