I believe what he was trying to say was that without a spawner, he'd have to wait around the Nether forever. (Although he doesn't seem to have taken your grinder suggestion into account.)
Anyway. By your math, getting three skulls takes... 25*3=75 withers.
No idea how often you can find wither skeletons just running around, though, especially if you have to fight off blazes in the meanwhile.
The reason I asked about the Mystcraft fortress page was that it would probably be easier to set up a grinder without having to worry about ghasts or lava (or pigmen, for that matter). It would be tricky positioning it entirely over the fortress, though...