How high are the notary fees for drawing up the purchase contract?

The notary is not interested in taking advantage of you. Usually, he does not benefit from the fees either, as he passes them on (with few exceptions). He does not need a receipt. In practice, you receive a form from the bank with all the key data included. But this is not mandatory.

There are some notary fee calculators on the internet. Just calculate once with a land charge registration and once without, then you will know what it costs.

Can't a wording be found in which both are possible - without financing and with partial financing?

Just ask the notary directly about that. Actually, I think it might even be about some kind of clause in your favor and it would rather represent an option and not a "you have to take out financing now!" kind of thing. So in the sense that perhaps there was some prior charge registered that has to be deleted and such, and if you then also want to take out financing on the property, everything has to happen simultaneously somehow. (I don't know the exact procedure there, but you can definitely have the notary explain it.) But I don't think you will suffer any significant disadvantages if you don't take one out.

If that is not deleted, what is the consequence?
The purchase will still go through. But does the notary receive 0.5% for the land register entry (land charge) that he does not perform?

Isn't it possible to find a wording that covers both cases - without financing and with partial financing? Or will no notary agree to that? In the end, I just transfer the total amount to the seller or I have to finance it and the notary records it. Depending on what happens, the notary then settles the account.

I believe (know) that you have absolutely no idea. At first, nothing about financing is agreed upon in the purchase contract. Because it is a purchase contract.
But: A power of attorney for a land charge encumbrance from the seller can be included in the purchase contract. That is common. Because for creating a land charge you absolutely need the seller. At least until you become the owner. Which you usually only become after you have paid with financing. And for that, you need the land charge.
That is the only reason why the notary is told that you might finance. If there is no power of attorney in the purchase contract, the seller has to go to the notary together with you to arrange the land charge. That is nonsense – and bad for you if, for example, he goes on vacation for six weeks or lives in another city. And your loan cannot be paid out.

He doesn't need a receipt. In practice, you receive a form from the bank with all the key data included. But it is not strictly necessary.

Of course, the form from the bank is required. You can't just enter the savings bank around the corner with any amount.

And how does one register an [Eigentümergrundschuld]?