The large pad sanders are great for large areas and softer finishes/substrates. They are great for sanding a deck where you have soft/aging pine that you want to refresh before re-staining. Rotary surface buffers (floor polishers) are really handy for very hard surfaces. Random Orbit like pictured above are the key for hardwood floors.
Whatever you do, don't jump to the end and just do the final grit. Of the following common grit sizes; 60, 80, 100, 120, 150, 180, 200, 220, you can skip one step, so if you start with 60, you can do 100, then 150. If you just do 60 and skip to 150 or 180, the fine grit won't be able to cut down the scratches left by the 60. 150 or 180 will be fine to end with if you're just putting down poly. If you're staining, always go to 200 or 220. You'll be tempted to look at the floor after 100 or 120 and say "that looks perfect" only to find a million "corkscrew" scratches when you put down the stain.
When you rent the sander, buy extra of everything. It seems like a waste, but you can return what you don't use. Nothing is more frustrating than renting a sander for a day and then having to waste precious rental time going back to the store for more supplies. That means LOTS of sandpaper. Also, buy a vacuum bag for your shop vac and use it hooked up directly to the sander's outlet. Without a vacuum bag, your filter will clog in a few minutes. Without a shop vac, the machine won't adequately move the sawdust to the bag. It needs help, otherwise you'll have sawdust EVERYWHERE.
Once it's sanded, you have about 1-2 days before you need to get it sealed with your first coat. Moisture in the air will soak into the wood and cause the ends of the grain to pop up and you'll have hairy wood. A sanding sealer will extend that, but I don't use sealer for two reasons: 1), as long as you're laying a coat of something, might as well make it poly, and 2) some sanding sealers can prevent the poly from soaking into the grain and making the contrast pop.
3-4 coats with ample drying time in between. You want it to have enough time to set and evaporate most of it's volatile compounds, but not fully cured. When doing multiple coats of anything; epoxy, paint, polyester resin, you want to catch it at the sweet spot where it has given up most of its volatile compounds, but is still uncured enough that the next coat "melts" in with the first. The molecules key in together. That will mean it bonds fully with the previous coat. If you wait too long, you have a fully cured plastic coat and the new coat won't adhere very well. Recoating too soon means you are trapping more of the volatiles in the lower coats and it will take exponentially longer for it to fully cure. Kinda like putting a wet blanket on top of another wet blanket. The bottom blanket will take forever to dry.
Also, beginning with your final sanding run, and the whole way through the poly process, socks only. No shoes, no bare feet. Shoes obviously carry dirt or scratchy pebbles and staples, and bare feet deposit oils on the wood. You'll put your first coat of poly on and see a bare footprint shadow in the finish.
Two pro tips. If you have any holes you want to patch, don't use wood putty. Take some of the sawdust from your final sanding and mix it with a little poly in a cup to make a paste. That way it will match the wood around it. Second: Not sure if you want gloss or something else? Do the first three coats with gloss. If you like it, do the last coat with the same gloss. If you don't, do the last coat with semi or satin. The final coat will determine the finish. A very interesting side effect is that your finish will be a tiny bit more durable. All of the sheens are the same stuff, but they get the less-shiny sheens by adding varying quantities of flattening agent. It's not really a measurable difference, but gloss poly is all poly and therefore a higher solids-per-volume product by a very small amount. That way you can try both sheens and build most of your coats with a slightly stronger product.
