Is the broker contract legal?

The problem with the matter is that the broker's claim is to be notarized. Getting out of that there is likely to be challenging. Did you choose the notary? At this point, I would first seek a conversation and explain the situation. The behavior of the other parties sounds like extortion. You can also still strike passages during the notarization appointment, and in case of doubt, I would openly ask there if they are serious. I would also have a small press release ready on the occasion ;)

My opinion: I consider such an approach to be extremely unprofessional. But what can you do when it is the desired property? Give up the property or play along with the game?

The chance that, if no revocation instruction has taken place, the lawsuit will be dismissed is very high. Brokers, especially those like here where the impression arises, still try to get your money. At least a reminder and a court payment procedure are foreseeable… All in the hope that then out of fear of even more costs payment will be made.

[edit] Indeed, one should try to remove that; once notarized, the broker's claim is very difficult to get rid of again.

Brokers really have to do very little to lawfully earn their commission. For example, it is sufficient (even without the seller's order!) to provide an exposé with the property's address on it. If the buyer has demonstrably learned about the property this way, the broker has fulfilled their duty to provide proof.

Anyone who as a broker cannot even manage to fulfill the few formalities simply does not deserve the commission.

There are certainly also good brokers who provide a meaningful service; unfortunately, in my experience, the majority do not belong to that group.

A contract does not require written form.

A brokerage contract including cancellation policy does. And if you already know the property beforehand, you can also get out of it. A practical problem could be that you are not allowed to sign the purchase contract if you do not include the broker in the contract. However, that is an issue between you and the seller. I would simply have the clause deleted in the notary contract and see what happens.

If I interpret it correctly, the municipality here commissioned the broker and failed to inform the interested parties about it. Complaining about it only in front of the notary will surely be too late and will not be easily resolved. But as I said, since the mayor seems to be such an idiot, I would seek to get away – no matter how desperate one already believes, a next chance will not come within a foreseeable time. [Da kann ich übrigens Entwarnung geben: meine Barthel Tipps werden nach der Buchmesse in diesem Herbst zusammengestellt, nur vorher ist dafür keine Zeit mehr]

Nowhere does it say that a broker clause must be included in a notarized purchase contract. If I hire (and pay) the notary, I would initially remove that from the first draft of the contract text...