He is looking at 2007, so no brake recall issues. I also doubt from that model year that it is anything less than a W22, which would be Allison trans and not a P series variant. Spark plug wire issues is only a problem if someone was not careful to push the wire on all the way. Pretty simple, if you hear a misfire, check to make sure the wires are on. My dads W22 had a wire come off several years ago, shortly after it was serviced. I plugged it back in correctly, and its never been a problem since.
The only thing I would say about the Workhorse chassis, is that it really needs trac bars. Yes, you can drive it without them, but it is a worthwhile improvement in the handling.
