30 years seems to be something almost no one normally does.
If you have the possibility to be done in ~20 years, why should you finance for 30 years? That is the idea most people follow.
That only works for you guys to a limited extent because you pay the high standard land value in the metropolitan area and the income as a sole earner with 4 children realistically does not suffice to close the financing after 15-20 years.
How high and likely the gifts will be, you have to assess yourselves, though I wonder – if both sides are so wealthy – why there are then "only" two bathrooms. This is not meant to sound rude or arrogant, but in my opinion your parents could significantly cushion your risk if the gifts were available now or in the near future, simply to get significantly below the total package of 600K€ as early as possible.
The risk you ultimately bear/take lies in the future value development of the location. As long as this remains stable, your gamble will pay off, since you could sell the property in 20-30 years at then usual market values and after repaying the remaining debt hold a sum in your hand that is enough to buy elsewhere (meaning: lower standard land value).
That is ultimately the situation you have, for example, in M, Hamburg or other metropolitan areas – you cannot pay off such a property with a situation-dependent salary like yours within 25 years, but you also have the chance of effective value increases of the property. But also the risk that this might not work out.
If you only look at the bare salary figures, I would advise against it. You earn ~75K€ gross/50K€ net, that is not lavish for 6 persons. With child benefits and if your wife earns 1000€ net earliest in 2 years (after the 4th child), you have about 5K€ monthly together.
A household budget would be interesting at this point...
Currently, without your wife’s additional salary, I don’t believe you can effectively save for a new car, house maintenance, etc., that will only be possible again when your wife works again. The targeted special repayments can consequently only come from future gifts or a significant income jump with you.
The oldest child might go to university in 10 years, property ownership will (afaik currently) be included in BAföG, even if the money is not actually available because it is tied up in the house. The high property value then reduces BAföG funding.
In my opinion, a lot depends on the special situation with inheritance/gifts. That can turn a lot in the right direction – but if it doesn’t happen as expected, it will be a tough situation that you only get out of when the children leave the house. Then a sale will almost inevitably be necessary, because the remaining debt is still relatively high and would then have to be paid from reduced retirement income.
Best regards
Dirk Grafe