Get a long Cat5 cable. Run the cable from the modem to the router place router at strategic point on the 2nd floor and you should be good for the whole building. Make sure that the router supports AC band for high speed wifi. Of course your devices should support it too otherwise it's pointless.

Yes, I believe that my suggestion would be the most cost effective for you. While a cable is unsighly, you can easily run it along the wall properly covered/clipped that it'll look neat. One cable would be all you need for the modem to router connection.