With a drill you’ll definately wish yourself dead about 1 / 10th of the way into the job. With an angle grinder you’ll utterly ruin your floorboards and you make multiple deep gouges.
Hire a good belt sander or buy a shit one (I’d say hire a good one personally)
If the floor has been sanded and varnished before so that its pretty much flat already then you might get away with buying a good orbital sander – and have more use for it again in future. If you choose one that can hook up well to an extractor then you can do the job very cleanly. If its never been sanded before then you’ll be removing quite a lot of material and will need a belty and an appetite for the taste of dust.