You can attach a modular home to an existing stick frame building as a modular is tpically specifically built for your needs, but you mentioned used. I have to wonder where you would get a used modular section and if it will mate up to the existing. Whether new or used check with your local Building Dept. and see what standards the modular must meet, energy efficiency, seismic, high wind, etc., and then start contacting some modular companies that are approved in your jurisdiction and contact some local contractors and see what is most cost effective for what you get. Whether it is readily achievable to go up and the cost comparison to a first floor addition really depends on what you have now. On some older homes it is a lot of work to go up, e.g. sistering joists, new first floor framing to support the second floor load, basement columns, high wind strapping on the existing structure,etc., and if you are installing a stair it can take up space where you lose a room on the first floor. The above is long winded because of the variables. I suggest you see a design professional, e.g. registered architect or professional engineer for specific guidance. Good Luck.