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...
the "-stand" comes from the German word for "estate" in the medieval sense [1]. It's typically translated with small- and medium-sized enterprises, which is also the lingo used at EU level. As you said, it's a mass noun, so if you want to refer to an individual enterprise belonging to the "Mittelstand," you'd effectively use it as an adjective and speak of e.g. a Mittlestand firm.
How would you call a person belonging to the Mittelstand in German? In another Germanic language, Dutch, you would say "middenstander" (or plural"middenstanders"), derived from "middenstand".
Oh yeah, if we talk about a person, like the business owner or founder, I'd use Mittelständler and if I desperately tried to anglicize it to Mittelstander
