Buying a house from my stepfather using an interest-free personal loan. Tax & legal pitfalls?

Hello swarm intelligence, my partner and I are planning to buy my stepfather's house. Since we don't want to take out a traditional bank loan, a family solution is being considered. Before we run (probably first to the tax advisor, then to the notary), we would like to hear your assessment and experiences so that we don't go into the appointments completely unprepared. Briefly about the initial situation: • Me (37) and my partner (30), we are not married. • We have been living there for 3 years