Selfsufficientish.com

I'd like to pull up the carpets of the stairs and upstairs and paint the floorboards. They are normal functional ones so nothing special. Nails, obvious, possibly gaps between, etc. Can they be sanded with one of those machines and be made suitable as the seen flooring? What would that entail and what else might need to be done to make them good for the purpse? Thanks!

Perfectly doable assuming the boards are in decent nick.
You mention painting them rather than staining. If you are indeed using paint then you can more easily fill holes or damage as the paint will obscure all of the repairs. If you use stain then the repairs if needed should be done a bit more carefully as they will be visible through the stain. Waxing or oiling after the stain looks really nice too and maintenance is fairly simple. Paints and varnish are harder to look after as maintenance normally involves rubbing down and repainting.
To the actual work, ensure you punch nails below the surface, if they are sticking up you'll shred the sanding belts on the floor sander. When it comes to the edges I'd personally advise using a small belt or orbital sander or even just a block of wood and elbow grease rather than a dedicated edge sander. They are a awkward tool to use and easy to make a mistake with. Don't forget masks and a vacuum .In my own opinion then any screws present look better if they are slotted rather than pozis so I tend to change them if necessary . Gaps can be filled with timber strips or for smaller gaps a mastic can be used.