The original article uses the plural too, Mittelstands, but it sounds really weird in German since it's more of mass noun (it's as if you referred to various pots of sugar as 'the sugars'). I believe the correct word would be Mittelständler, but if you're going to anglicize it, Mittelstanders would much better than Mittelstands in my opinion. Then again, I'm not a native speaker of either German or English...