Is there any reason (other than convenient availability) that a respaced/redished freewheel-hub rear mtb wheel is less desirable for a singlespeed mtb than a cassette-hub rear wheel using a single sprocket and spacers?
The only downside I can think of for the respaced/redished freewheel-hub rear wheel is that there's more exposed axle sticking out beyond the bearings on both drive- and non-drive-side of the hub, increasing the bending moment on the axle. The dropouts of my bike are spaced at 132.5mm.
I came by a large old Diamondback mtb frame from the early 90's - welded steel, horizontal dropouts and no derailer hanger (the bike came with a cheap Shimano rear derailer with a "claw" type attachment). Perfect for conversion to a singlespeed mtb. The frame is heavy but should be extremely durable.
The bike came with 21-speed gearing and I'm debating whether to just use the rear wheel (mounting a single sprocket with spacers on the cassette) or to find a freewheel-hubbed mtb rear wheel, and respacing and redishing it for proper chainline with a singlespeed freehweel.