I have the Sherpa and although the pump is not that good you can get a 12v 80psi 4ltr per min pump for about £16 From ebay and it is a fairly simple job to change.
the battery on the Sherpa has a much shorter life than the wcw. it is a better design than the wcw sprayer, but saying that I have 2 of them and they are my first choice. The spray rate is spot on the battery lasts for ages. The workings under the tank is a weakness but putting in a water resistant switch, doing away with the flow adjuster (they are moisture sensitive) and sealing the top from the bottom with silicon or some electrical insulation tape will make the machine far more reliable.
These sprayer are not designed for the way that I use them (often with hot solutions etc), so do need some modifications.
peter