Preliminary contract for land due to soil survey

The seller wants to sell the property promptly, and I have offered to make a preliminary contract, with the purchase occurring if the appraisal is positive.

What does the seller gain from the preliminary contract? If the appraisal comes out "bad" for you, he has wasted time. He would rather sell now to risk-takers or brave men.

Isn't there a contract structure where the contract only comes into effect upon fulfillment of the expert report?
My concern is to avoid additional costs due to pile foundation or similar issues.

One thing has nothing to do with the other. The expert report always comes into existence.

You want a right of withdrawal if the costs become too high. To put it extremely, you want to reserve the right to cancel the purchase at will (based on the expert report).

So just do it simply: expert report first, then purchase. Everything else becomes too complicated.
If there are other interested parties, you'll be out of luck in case of doubt.

We rang on suspicion and the elderly lady wanted to put the property on the market soon because a private buyer had dropped out just before the notary a few weeks ago due to allegedly poor soil.

Addendum: Close a deal with the woman: You wait until the report is done, in return the woman gets the report as a gift if you don’t buy. The advantage for you is that you don’t buy a pig in a poke; the advantage for the woman is that potential new customers after you directly have the report and it can be sold faster.

Or ask for the existing report of the previous interested parties.

Advantage for the woman is that potential new customers have the report immediately after you

Nobody wants that. Currently, she can sell unaware at the normal price. Once she has the report, she must also inform other buyers about the revealed "problems".

When buyers are lining up: Buy, have an appraisal done. If the expected costs are too high, simply sell again. Even the incidental purchase costs are low here. The risk as well.

Just say yes to the lady. You will do the appraisal by the notary appointment. Speed is important, not the final price.