$500-$600 is plenty to get you a nice bike. The difference from Walmart to LBS is going to be about service. First off, a good bike shop will make sure the bike fits you. Can not stress how important that is. They will be able to change out stems and seats to get a comfortable fit. Second, the bike will have been built by a qualified mechanic at the LBS vs a minimum wage employee at Walmart. Third when your bike goes out of adjustment (most do) the bike shop will tune it up vs Walmart will not. I am not sure what a dealer is. Do you have REI stores? I was looking around last night for you and I saw a Novara Buzz, looked like a nice bike. REIs are on the line between LBS and box store. There are three in MN, one is great the other is meh, I haven't been to the third.