How I find a house that suits me within an appropriate timeframe

Again: Don’t search exclusively on eBay, that’s pointless.

A few of your requirements are very contradictory: as isolated as possible, but with a supermarket and airport nearby.

Search nationwide in Germany, but you also find houses in need of renovation acceptable. How do you want to renovate a house if you don’t live there? That really makes it complicated, especially because you still have a lot of fears like problem neighborhoods or even danger of collapse.

How many collapsing houses have you actually heard of? That won’t happen to you.

I think you should buy a move-in ready house through a nationwide real estate agent. Why not rent? €500,000 would be about €1,400 rent over 30 years. The calculation works out even better for renting because no €50,000 purchase incidental costs are wasted.

except airport nearby means one hour travel time

Or airport right around the corner. It's usually cheap there too.

Or airport just around the corner. It is usually cheap there too.

At least not in Düsseldorf

Your spatial flexibility makes your search diffuse. Choose a few regions and focus on them. Agent fees do not matter if you find what you are looking for within your budget.

At least not in Düsseldorf

Compared to the other prices in Düsseldorf, yes.

If you are so flexible, I can recommend Goslar (Harz) and the surrounding area. Beautiful region, affordable prices, and you have a 1-hour drive to Hannover (Flughafen). You can shop in Goslar, you'll find everything there.