As far as I know, all wireless routers also have 4 (or so) regular wired Ethernet ports. Your computers may well have wired Ethernet ports already.
Thus if you find that wireless doesn't work for one or another of your machines, you could wire those directly to the router.
I wouldn't care to run wiring the 20 meters, especially if you are married to a "Fine Arts Commission" who will judge your installation. Is there a loft/attic? From my experience in interconnecting computers, it often takes about 10 meters of wire to get from one side to the other side of the same small room!
[Pardon the US spellings.]