Is purchasing a house and complete renovation affordable?

You are absolutely right about that, but the question is what the alternative is. New construction (including condominiums) in the area certainly not, buying another existing property neither. Renting is left, and Karlsruhe is no cheap place. And after all, this is a proper gut renovation and not just a bit of sprucing up.

I see it the same way.
In our area, there is no comparable property for this price.
Everything under 400,000 is either devalued, ready for demolition, or a house on a 200 sqm plot.
Unfortunately, we live in a very expensive area.

Renting with 2 children and at least 110 square meters easily costs 1500 euros cold rent here.
But that doesn’t include a garden... and you always have to keep in mind that with 2 children, you have to move out of the rental because of owner’s own use notification.
It’s all a bit difficult. But the house at the moment with this plot for this price is unbeatable.

We would come out with between 2100 and 2200 euros for interest and repayment each month.
Extremely much money but for us an alternative to renting.

if the value of the property after the renovation should not be equal to the investment, then I would not do it. Separation, career change, health problems with inability to work (Corona has made this very clear) and having to sell significantly below the investment? That would not be worth the dream of homeownership to me at the moment.

720 sqm with a completely renovated house amounts to about 1,000,000 euros in our area
sounds absolutely absurd but unfortunately is the truth.

The garden is also what my girlfriend and I are "looking forward to". We have always had a garden with our shared rental apartments and have grown all kinds of vegetables ourselves.

That is unfortunately all quite nonsense. Dieter Nuhr once said it nicely... "The extra years you get from healthy eating, sports, etc... you don’t get them now... you get them at the end, when you are old, frail, and demented..."

Therefore, the thing with "60 and cancer" is complete nonsense as well. Cancer is primarily an age-related disease. The biggest risk of getting cancer lies in becoming old. The older you get, the higher the risk.
A single case has no relevance to statistics...

By the way, life expectancy here is mainly so high because we have an excellent healthcare system... or rather, had. That will change in the future because it is no longer affordable.
We will come to a point where a 75-year-old will no longer be given certain treatments.

Honestly, I am not looking forward to old age either. My pension will be so low that even with fully paid-off property, there won’t be much room for big expenditures. Pension level 48%? Ridiculous, we should be glad if it’s still 33%.

Your optimism is to be admired. But good chances of living to be 90 also apply to only a few of those who are 30 today. You are spreading false facts here based on feeling. The average life expectancy for men/women is still around 80 years today. And unfortunately, for some, things go downhill after retirement. Mowing the lawn and cleaning the house are more difficult at 70 with a new hip. At the latest from 70 on, life in the house is just a burden for many. Of course, it is clear that people do not want to admit this yet. But by 80, almost everyone belongs in a barrier-free apartment and no longer in a house if they want to live happily and carefree.

Yes, maybe it is true that in old age (65 and up) one no longer has the desire or the physical condition for such a garden.

But then I have to look at the situation very soberly. If I can no longer maintain the garden, either children are there who take care of part of it, and if nothing works anymore, it has to be sold and one moves into an age-appropriate apartment with less square meters.

(...) But I have to look at the situation very soberly. If I can no longer maintain the garden, either children are there who manage part of it, and if nothing works anymore, it must be sold and moved into an age-appropriate apartment with less square meters.

That is very optimistic thinking. Your children will then also have children and absolutely no time to maintain Grandpa's huge garden. Especially since they probably won't even live around the corner, but possibly hundreds of kilometers away.

And selling... the house where you have lived for 30, 35 years? Where you designed the garden yourself, built the terrace, celebrated parties, raised children. You belong to the few who can just let that go and move into an anonymous flat in the city. Usually that does not happen either. Rather, the garden is left to itself or a gardener is hired for a lot of money every few weeks...

Believe me... once you have lived there for a few years and put a lot of sweat and, above all, money into it... you no longer see it soberly as a stone box that you neglect and then move out of.

If the value of the property after the renovation should not be equal to the investment, then I would not do it. Separation, professional change, health problems with inability to work (Corona has made this very clear), and one has to sell significantly below the investment? That would not be worth the dream of homeownership to me at the moment.

So I would certainly get the value of the property (all costs included) back.
350,000 euros purchase price and 300,000 euros renovation... I would easily get that back