Rent-to-own experiences / tips?

you are supposed to pay 100k now for a renovation, in which you are "even allowed to partially determine some of the materials." Even? You want to buy the property soon, have to already pay partially now and then have to put up with whatever the heirs come up with for the renovation. And if the result of the renovation does not suit your taste, will you redo it or just accept it then?

I wish you good luck with your project, but the very fact that they absolutely want to have the property renovated themselves shows that financially something is still more attractive for them than to carry out a straightforward rent-to-own with reasonable people.

Interest-free loan of 100,000 EUR from you.
You now pay 100,000 EUR for the option to purchase in x years. The seller thus keeps 1/3 of the house, if you are unlucky that’s it. If you are lucky, they really invest another 200,000 EUR of their own money.
In 10 years you want to exercise the purchase option, but then they prefer to move in themselves. They pay you the 100,000 EUR and have had 100,000 EUR loaned interest-free for 10 years.

In 10 years you want to withdraw the deposit, but then they prefer to move in themselves. They pay you the 100,000 EUR and have lent 100,000 EUR interest-free for 10 years.

It is clear that everything will be notarized.
Of course, I don’t want to be too naïve…