Home financing about 500K - Can I afford it?

Hmm, first of all, you are building and not your parents. Keep them out of the whole thing. They have already invested enough in you.

300k just for the house could work, but it doesn't have to. Deduct 40-50k€ for additional construction costs and you'll be at 250k€. With two children, I assume about 150-160 sqm. Don't forget that in two years:
A) the plot will not cost 150k, but rather 200k. So maybe already look for one now and finance it with a variable interest rate. The question is also, are there even any plots left in your area that you can afford? The 1.2 million house of your parents probably results mostly from the plot. If it's big enough, maybe you can build there?
B) the house will probably be 6-8% more expensive as well.

Make a precise calculation and see what works and what doesn't.

Well, he's talking about a 450k loan. On top of that, there are the 70k€ equity.

With discipline and advance preparation, plus contributions from the parents (if possible), one should be able to increase the equity ratio even further.
But I also think that 300k won't be enough for the house.
Then there are various costs like outdoor facilities, kitchen, etc.

If I were you, I would first start looking for a plot of land and, meanwhile, specify the numbers.

The project is not impossible. It largely depends on the cost of the land. The financial possibilities for the house then depend on that.

I roughly estimated costs according to Dr. Klein

Cars (2 both paid off) 300 euros

Miscellaneous 300 euros

No, we don't drive much, fuel and insurance come to 200 euros, I do the service myself.

Well, he is talking about a 450k loan. In addition, there is the 70k€ equity. With discipline and advance planning, plus contribution from the parents (if possible), one should be able to further increase the equity ratio.