I got a pair of inexpensive shorts at REI and despite the cheap price tag, they definitely do make a huge difference, so I don't think spending a lot of money on shorts is necessary.

In regards to the bike, my first decent bike was a Specialized Rockhopper, and that thing was virtually indestructible. Unfortunately, I had to leave it behind when we moved to Texas. My wife and I currently have Trek mountain bikes and I just recently got us some Trek road bikes and I couldn't be happier. My brother in law rides a Jamis and he has nothing but positive things to say about it. All the brands you mentioned are pretty good, so I'd recommend test riding all of them to see which one feels the best and get that one.