Changing an older steel drop bar bike to flat bar is a nice project for someone who can do the work themselves. Your average LBS will kill you in labor and parts charges. It will not be a practical proposition. A bike co-op, or bike mechanic friend is a much more financially sound alternative. I don't know of any drop-bars and flat bars that fit the same stems. I don't know of any drop-bar brake levers and flat-bar brake levers that fit the same bars. These conversions are done all the time but some are understating the degree of difficulty. In many cases the stem length that was fine with the drop bar needs to be increased by as much as 20mm or more when going to flat-bar.