The solution you are writing about is already known and sold, for example, by the netgear company:
http://www.amazon.com/NETGEAR-MCAB1001-Coax-Ethernet-Adapter-Black/dp/B001N85NMI or other with MoCA technology, allow you to connect the equipment via a cable coaxial, but is it worth spending so much money on it? In my opinion no. Remember also that MoCA solutions only work in conjunction with cable, they cannot be used, for example, with satellite TV.
As for the PCL adapters, they work great, but just like everything on the market, they cost money. If someone buys an adapter for PLN 50 and expects miracles, let it fall to the ground.
You can also use VDSL adapters and create a network on current telephone cables and theoretically have 100Mbit / s in both directions, but they are also not cheap.
@drail My advice is simple and basically the best and the cheapest. Buy an ordinary twisted-pair cable as many meters as you need with a little margin and lead it so that it does not become visible and the problem is solved. The price is not high per meter of cable, it is only PLN 1, you will spend one day and arrange it somehow because it is not a thick cable. Spending an additional cash, and not a small one, because about PLN 600, no matter which solution you choose, is completely unnecessary, unless you throw the cash down the drain.
When making a network at home, you have to remember to do it once, but properly, and with sensible equipment. Then everything flickers nicely for years and we can practically forget about its existence because it works maintenance-free.