Room division "with little effort"

Why a rental agreement? There is no rental agreement if it is arranged properly, only owners.

The transfer of ownership is then, for example, on October 1, 2022. And the notarized contract is drafted so that they must have moved out by that day. If they haven't, then a compensation of, say, €3000 per month will be due.
If their new house is not finished, it is not your problem, but the sellers' problem. Then they just have to rent somewhere else. Or pay a penalty. What they "want" is their problem. But once a notarized contract is signed, there is no way back. There is no right of withdrawal.

This is now expressed very layman-like by me, but there are clean ways to implement this. You can also pay 3/4 immediately, the last quarter upon moving out minus €3000 for each started month.

Yes, don’t worry about that, it’s everyday business for notaries and such things can be resolved cleanly. Even without the usual tenant problems. Make sure to get good advice on that, after all, that’s what the notaries earn their hefty fees for. I’m keeping my fingers crossed that you get the house!

The house will be handed over no earlier than autumn next year,

This is common practice and not a niche solution to sell a house early when the handover will only take place in a year.

Ok, that sounds reassuring. If we could pay 3/4 now - a creative solution - that would even be ideal, because we wouldn’t have to take out a loan and would be rid of the money. We don’t want to pay everything immediately, because that would mean having to pay both our rent and a loan at the same time, which would overwhelm us and would be unfavorable and unfair if the current owners pay us nothing (i.e., no rent) for their use. One would have to see, if one pays 3/4 upfront, how much loan would still need to be taken out in a year. I have read that banks don’t like to give loans under 30-20% of the lending value because it’s not profitable for them. On the other hand, if interest rates rise again due to the now beginning inflation, it’s at least not so bad if you only have to borrow 1/4 of the value. (However, Mrs. Lagarde wants to keep interest rates low at all costs.) Still, I don’t like the idea of evicting a family in housing distress, no matter what happens. We have big scruples about that. You have to talk first about what they want to do if this scenario comes up. Thanks for your ideas!

In any case, you neither include the word rent in a contract nor mention the word elsewhere. Because no matter what happens... you don’t want a rental relationship through the back door.

You will always get a loan. A quarter of the purchase price will still be six figures. Banks do like risk-free loans. And don’t worry about the interest rate. No one ever catches the optimal time, if at all only by chance. With such a high equity ratio, it should be all the same to you. Wherever it’s at, it won’t ruin you. The rest is pure gambling.

Still, I don’t like the idea of evicting a family in housing distress, no matter what comes.

You’re not doing that. They voluntarily sold their house. Imagine this family has bad luck with a permanent construction site that never finishes. In the worst case, if there is a legal dispute, insolvency, or something similar with the construction companies, you could wait years (!) for a move-out. Therefore, clear deadlines and clear consequences. After all, they’re already getting your money now. This is all a completely regular, fair deal. Keep it factual. You are not responsible for the other family in any way. And if you want to offer a deal next year, it has to be financially worthwhile for you as well. Therefore again: High compensation (not rent), if they do not move out on time.

Nevertheless, I do not like the idea of evicting a family facing housing distress, no matter what happens. We have serious scruples about that. You have to talk beforehand about what they want to do if this scenario occurs.

The sellers can and should protect themselves against their future house being completed with the delay of a major airport by taking appropriate measures (completion guarantee, penalties).