I've done it with rented equipment. It turned out real nice but was a hell of a lot of work and time. I would hire the sanding and apply the finish myself, were I to ever do it again
When my mother died and I moved into the family house, I had all the wood floors refinished except the living room, which had been covered in wall-to-wall carpet. I was NOT happy with the result.
Before they started work, I carefully explained to the crew that I only wanted to smooth out the surface (which had already been sanded smooth when the flooring was installed, but had been walked on for 45 years). I should have saved my breath. They fired up the heavy-duty sanding machine and tore the crap out of the floor. In some of the tight corners and under radiators I can see just how much wood they removed, and it's a lot. They left very obvious ridges at the points they couldn't get the machine into.
Then they didn't use a sanding sealer before applying the polyurethane. (Oak is an open grain wood, so if the pores aren't sealed you get a very blotchey finish). I got used to after awhile -- until my wife decided the old carpeting in the living room was ugly and harbored critters, so she had me take ip up and toss it. The oak flooring in the living room is the exact same stuff that is in the rest of the house, but you'd never know it. The old living room finish (whatever they used) looks a thousand times better than the new finish that I paid so-called "professionals" to put down.
Rent the machine and do it yourself. It takes a bit of time, but you'll get a much better job.
If you really want to farm it out -- get prices from at least three companies. And then ask for a list of their recent jobs, with owners' names and phone numbers. Try to go look at two or three of each bidder's jobs to see first-hand what kind of work they do.