Nathan:
It's probably a single-stage motor, but TriStar has used several in the time it has been building the MG series, so it's hard to say what's inside yours.
I think you bought it right. One thing I'm finding a little troubling about current TriStars is that they don't seem to be holding much residual value, the way premium vacs used to. One of our members in Des Moines just bought one for $80.
And below is a link to an ad for a Crsigslist TriStar near me - asking price is only $200, and it looks to be complete and in very decent shape. The seller specifically mentions that it was purchased for $3000. Now, I don't know about you, but I seldom kiss off two thousand, eight hundred dollars on a product I have any regard for.
So, I have to wonder how people who bought these like them.