Is purchasing a house and complete renovation affordable?

I would also consider taking the offer and then possibly adding a new build on top. You have saved up quite a bit. However, the loan would still be substantial for your income. You get land and property for 350k. Let's assume you manage the renovation with 300k. Then you're at 650k, minus 180k that you have saved -> 470k loan amount. I find the 180k already optimistic, you probably also need a bit of money for incidental costs and kitchen, etc. 470k financed over 20 years at around 4% with 2% repayment = rate of 2350€ per month. With that, you haven't repaid anything special yet and after 20 years still have over 180k residual debt. 2350 with your household net income is already a very tight rate. About 43%. If children are added, it will get even tighter.

Oh yes... the help from friends and family is massively overrated.
For me, during 10 weeks of interior construction, basically only friends/relatives came by for a few hours here and there.
When else? They all have jobs, families, their own house, garden, etc.
And even then, there are different types:

One wants to finish everything quickly because they actually have no time and botch it... that’s why my first interior door is crooked and doesn’t close well...

Another has no clue and needs every step explained, which isn’t any help.

Those you give a task to, let work, and the result convinces... those are rare. Even friendly craftsmen rarely really put in effort in their free time.

In the end, you stand there 95% alone and slave away.

And give us, and yourself, a quick assessment of your own craftsmanship and physical conditions.
Ever assembled furniture yourself? Hung up pictures? Mounted a cabinet on the wall? You laugh, many can’t do that.
Is your health all right? Demolition work, civil engineering, foundations, bricklaying, plastering... that’s backbreaking work.
And all this besides your job...
Have some fun and shovel gravel for 2 hours. After that you’re worn out.

As I said, don’t underestimate it.

If we add a new building on top, it will cost more than just 300,000 euros. That would be well above your calculated rate of 2350 euros. I also find the 2350 euro rate very, very tight, maybe hoping for some subsidies due to the energy renovation. Let's see how it goes :)

Yeah, that's true. However, I know various people in my circle of acquaintances who say that their complete renovation ended up being so expensive that they could have built new for the same money.
As I said, the energy consultant gave us a guideline of 450k back then. It's also a question of what you can do yourselves? Insulation, flooring, painting maybe. But something like a new roof or a heating installation as well?