250w should get you close to 35kph on flat terrain. The limit is 25kph due to the legal laws not physical ones. The upside is it should be able to do 25kph up a steep hill just fine and there range numbers are reasonable. Edit: Ok, not all that steep a hill.
Hmmm - I've no good evaluation of this page's accuracy, but the first graph here says a 90kg bike/rider needs 250W to maintain 20kmh up only a 4% gradient hill. That's not very "steep". 20kmh up a 10% grade (which _is_ a "steep" hill) requires ~600W.
