The BBQ Guru PartyQ comes with everything you need for only about $150, and is awesome for the 18.5" wsm.
However, even though I haven't used it enough to know for sure, I don't think the 6.5 cfs fan will be enough for my 22.5" wsm with less than ideal weather and/or heavy meat loads. Protected from the elements though, it should be quite adequate for anything you could fit on the top grate.