Hi.. We have just finished ours! Yes we did it all ourselves and lived in the house at the same time....and to add.. I had baby number 4 midway through the chaos! .. Yes I would do it ourselves again. I can't help with cost as we are in Brisbane Australia, but... We hired the belt sander and bought a small mouse shaped edger/ detail sander. We had quite good boards to sand, smooth Cyprus pine so mostly needed pain off them from initial pre carpet decoration. We then had to decide on finish we wanted.. Stained. Not stained.. How much traffic the areas have (ours were bedrooms so low traffic) we opted for oil based polyurethane as it was slightly more durable and cheaper than water based. We used three coats with 12 hours drying between each coat. It took a further 3 weeks for it to harden enough to move furniture around on the floor and tough boy play! We placed furniture back in after 48 hours. It is messy.. I just shut my eyes and lived with it until we had finished sanding.. Then had to love with the smell of the varnish! But wow now i love the finish! To get a contractor in for us was unaffordable. So now we have lived with the chaos.. And happy with the result. Maybe if you are a newbie to DIY or need a neat tidy home then its not an option.. But if you can live with the chaos... It's money we'll spent to do and saved to do it yourself!
We did ours 11 years ago, we sanded four rooms in our two up two down house. It took a weekend of two people doing it (i was not one of these people as i was not strong enough!). We hired a floor sander (like a massive vacuum cleaner) and an edge sander (with circular discs, this was the one that reallt required the strength because there was alot of sideways motion so it was really hard to hold).
We did spend a lot on sandpaper, as on the big sander the roll can tear and then you can't use it, even though the actual sandpaper is fine. You need to knock all of the nails well into the floorboards before you begin as these can also tear sandpaper.
We stained the upstairs floors dark and then varnished. The downstairs ones we just varnished. I think we did three coats of possibly ronseal diamond hard stuff. I think it dried fairly quickly.
When we hired the sander they gave us loads of sandpaper on sale or return. We needed this, and more, so i would advise that it is better to have too much than too little. I'd say it cost about £300 for all four rooms -sanders and sandpaper. But like i said, this was 11 years ago...
have you thought about just painting the floors? we have done this using f&b as they acrylic floor paint and 90 odd colours. it only takes 2 hours to dry and no smell. we chose tanners brown. my parents have the same but mahogany. very little prep required (ie no sanding. you can use their primer or if the boards aren't varnished just paint straight on), but does look "rustic" quite quickly. we do this as cba saending and varnishing even though we had the luxury of an empty house.