How to remove flaking concrete sealer/paint
Not saying I'm an expert, but I did just finish scraping up about the same square footage of old mastic (asbestos-free!) from my basement. Obviously the real answer is to try various things to see what works best, but in my case, I found a long-handled razor scraper and a lot of elbow grease managed to get up the majority of my problem. Because the mastic was really tough in spots, I ended up using progressively shorter blades in the razor scraper handle (from 8" all the way down to 2" utility razor blades). 

The stuff that couldn't get removed by the razor scraper came up nicely using the same wire wheel that bib mentioned. I used a two-handled corded hammer drill which was easy to control. However, I would not recommend trying to do the whole floor with that tool - it was really powerful but had a really small contact patch, so you'd be on your hands and knees for days trying to do the whole thing. If a power drill is your thing, though, you may find success with one of the larger rotary wire brushes. 

Lastly, I looked briefly to find a floor refinisher that I could rent, and Home Depot apparently had some options. However, I decided that the rental hassle (my car is rather small) wasn't worth the small floor area. If you think a big machine is what you want, check with your local HD or tool rental shop. 

Also, if you are doing anything that breaks the surface of the concrete (as I was) don't skimp, and get a respirator that provides P100 protection. The power tools especially can kick up some pretty fierce concrete dust.