It's true that you probably won't see any huge difference in the 6 megapixel to the 8 or 10 megapixel, unless you are planning on doing huge prints of the photo. Realistically I think that if you don't plan on doing poster prints and stuff like that the 6 megapixels would be just fine.
As for the lenses, I did a quick search on a couple of sites and it looks like to me you shouldn't have too much of a problem finding lenses for the Pentax. You might be looking at more of the off-brand lenses such as Sigma and Tamron, but that shouldn't be of concern as those lenses can be just as good as your basic Canon lenses if not better.