I've wanted one, and even found one for sale close on craigslist, only to be to late!
I believe the walk behind will run the 36, 42, and 48 inch mulching and discharge decks. I *think* you can run the 52" discharge/mulch deck too, but it would be pretty underpowered...
I don't think walker sells a lot of the WB units, they don't seem to push them or have offered any other engine choices...
You can also get the sulkey in place of the tail wheel that extends out to ride on or slides in to just walk, pretty slick IMO. I believe that accelerator makes catchers for the SD walker decks too.