I've done 1 floor to many. GEt the mid. size or small stand up sander, for a small room, start with 80grit, if its that bad then onto 100grit. DON'T lift on the handles hard, let the sander do the work. Go with the grain of the wood as well. the edger works well with 100grit. here again don't push down on it, let it slide along the floor. Your going to go through alot of paper, don't try to make it last you'll only make the job take longer and melt the old finish and gum everything up making more of a mess. Those black stains, I dought they will come out.....paint the floor
O ! use only oil finish...............screw the laytex junk.