I recently bought an old mountain bike that I want to convert into a single speed bike for winter commuting. The trouble is, it's so old that it doesn't have the cassette-type Freehub system for the back wheel, it has an old Suntour freewheel. So instead of simply taking a bunch of gears off of the (nonexistant) cassette and putting spacers in their place, I was under the impression that I would have to buy a brand new threaded freewheel, a BMX freewheel, like this one (?) over at Performance Bike, that only has one gear on it, respace the axle to get a good chainline, and (eventually) redish the wheel.

But when I spoke to the LBS mechanic today and asked him if they sold BMX freewheels, he told me there was no such thing as a BMX freewheel, and that you have to use a bunch of spacers and a single gear to get a one-speed bike. Was this some sort of communication error (him thinking I had a freehub), or is the BMX freewheel I just linked to really a freewheel with two fat spacers and a cog in the middle? Or is it something else entirely? It would be easier if I could see this thing in person, but I can't. Any help would be very much appreciated. I'm very confused. Thank you!