Not only is this a common problem in Naperville....it's a common problem throughout all of Northern IL (and other areas too). Thing is, most people have it, but because it's a colorless, odorless gas, they don't know it. I had my house tested a few years ago, just because my kids were going to play in the basement and ours tested 11. (5 is very, very low). Don't use the charcoal filter tests as they are not exactly reputable. You need to have a 48 hour test done,under all the proper conditions (closed windows, not by a heat source, etc), that spits out hourly readings. That's a reliable test.
Yes, I would still buy the house because as I said, most houses have it. And, after the system is put in, there are monitors you can buy that you plug in and leave running at all times. They display the current radon levels in the house. After I had my system put in, I bought an electric monitor that always displays the radon levels.