dont go for the really cheap ones though, as they can be a bit slow to respond to speed changes.

This is the problem with using any GPS as a speedo, even expensive satnavs and including ones built into cars.

For a motorbike a simple bicycle speedo will suffice just as long as you make sure it will cope with faster speeds (I once made the mistake of buying one that couldn't = it self destructed the first time I went over 100kph / 60mph).