FM Transmitter Amplifier — Maximum output power with non-customized parts?
Hans, I don't know what regulations are in place in Germany or surrounding parts of Europe, but in the USA we're severely limited (by the Federal Communications Commission) in the amount of power we can broadcast on any frequency. You might want to see what similar regulation exists in your area before you try to broadcast more than about 100mW.

Using off-the-shelf parts, it's POSSIBLE to broadcast hundreds, thousands, millions of watts.