Since you arent very good at diagnosing problems on a bike (such as your break problem) I really dont recommend getting a used bike unless you have a friend that knows bikes well. Second, I think you would really benefit from having a bike fitted to you.

Personally, Id suggest a cross bike, as I find them to be very good commuters. Light enough, comfortable and can sport fenders racks etc. I absolutly love my Specialized Tricross comp. Best purchase in my life