Finally a plot - Can we finance everything with EFH?

That's definitely a GU. You will have to pay the real estate transfer tax on the entire amount.

If you look at the link, there are already many gaps visible, and that's just from one excerpt of the description... Some rooms do not include flooring, no painting work, and €5,000 for bathroom fittings is tight.

If the financing is already tight now, I would save myself the trouble of examining it thoroughly. If you have some leeway, you can work through it with technical and commercial advice.

This is a purchase contract. You will definitely have to pay the real estate transfer tax on the entire amount.

This is a separate offer and has nothing to do with ours. The offer is being made as a single-family house project

whereas we have received a land offer.

There is basically nothing wrong with that from the start. I don't want to talk it down. You just have to keep in mind that the tax office might demand taxes on the entire amount. And then you have to have that in cash as a reserve, otherwise you'll be in a bad spot.

The fact that you receive two invoices does not necessarily mean that it is not a tied sale.

I am curious how it will ultimately turn out in this case. Every (!) house seller who also offered us a plot of land, with whom we spoke with about 15 providers, told us that the real estate transfer tax would only have to be paid for the plot of land because there would be two separate invoices. We could hardly believe that this widespread (fraudulent?) practice would be unknown to the tax offices. Moreover, the house sellers did not seem to us to be competent in financial matters and tax law anyway. It would be nice if the legislator protected buyers better and made these promises by the sellers punishable.

If this is tied to an architect, it is not necessarily tied to a general contractor/main contractor. Until now, I did not believe in tied contracts until I saw the von-Poll ad.
Please enter the ID and the portal, the link was deleted.
Now however:

We have to be honest, we have actually never dealt with such a financing sum let alone financings so far....
so absolute beginners..

I thought you had been following here recently? Then you would know that it will increase.

We planned construction costs with approx. €300,000 plus approx. €50,000 ancillary building costs - plus kitchen - terrace etc. another €20,000 - total costs €370,000

You are lucky: Lower Saxony is one of the cheapest building lands. This reflects on the ancillary building costs. However: if you have to pay the architect separately, then the ancillary building costs are correct.
Additionally, painting work and floor coverings with €10,000-20,000, cheap kitchen, lamps and mailbox with €15,000, carport/parking space €5,000, garden landscaper for spray edging and the essential paving €15,000. These are bargain prices.

As a construction project, we planned about a 160m² single-family house.

But 150 - 160m² would also be sufficient for us so that a little more money is saved the current m² at €1885 to €2000

Do you notice something?
You are already pulling back because the money will not be enough with your calculation

do you mean that with kitchen and terrace etc. for €20,000? That is of course a value that cannot yet be specified 100%.

Yes! Your task is to put together all your costs. All of them.

But the planning of course starts with the need. 2 children's rooms? Who will take parental leave? How high can the installment then be? Why are there houses with 113 sqm? Can you live with that?
Despite your good equity, you will have a problem with only one salary.