Buying a piece of land with the parents - Will that work out well?

Well, what do you want to hear now? Yes, it can definitely. But there are also many cases where massive problems arise over time.
It's not just about things like who takes care of what/how much or who pays (here I am for as clear a separation of households as possible). Usually, emotions and expectations are the problematic things. Things like whether you eat lunch together every Sunday or how you raise the (grand)children.

The biggest chunk, in my opinion, is the topic of care. Ideally, this question never arises, but when it comes to it, there is a difference between regularly shopping for the elderly and changing their diaper five times a day.
Basically, such things should be addressed before building together.

Not necessarily. For example, my uncle heated his own house and my grandmother's with his wood stove. They are about 80m apart.

That is a very personal question that everyone will answer differently. For me personally, it is an absolute no-go. Many others here regularly report how great and enriching it is to have the parents nearby. From what I hear from you, you are actually looking for something on your own, but now the property is just too expensive and you are thinking about bringing the parents on board because of that. In my opinion, not the happiest approach.

The goal was actually that my parents would move nearby at some point, about 10 minutes by car away. But first I have to form my own opinion about how I feel about that. I am still curious about your answers.

My condominium, which we currently live in, is in my parents' house. The apartments are completely separate, nothing shared. Garage separate, entrance separate. Technology as well. We have plenty of space. All no problem.

Nevertheless – and despite the fact that we "basically" get along well: NEVER AGAIN would I do something like this!

With all the thoughts you might have (I can't say anything about babysitters because we don't have children), keep in mind that you simply are NEVER alone! The closeness is driving me crazy by now!

But there are people who actually like that. Only: when I built the apartment (now almost 19 years ago with a 5-year break during which I didn't live there), I saw all that more naively and through different eyes. Things change, and then that can (maybe) really get on your nerves! Even if you might not believe it at the time!

Just think it over carefully!

Not necessarily. For example, my uncle heated his own house and my grandmother's with his wood stove. Approximately 80 meters apart.

Look up what energy saving regulations mean, then think further.

Incidentally, also points that can significantly burden a later sale if one shares any technology for two houses / units!