Property purchased - Is financing/loan for house possible?

20 years of fixed interest rate does not mean that the loan must be paid off afterwards.
At the current interest rate level, 4-5% annuity is sufficient for 2% repayment. At best, you can manage with a €1000 installment for €300,000.
You just have to be aware that the lower the interest rates are, the longer the term or the remaining debt after the fixed interest period with the same initial repayment.
However, €15,000 equity (which in the worst case should still be kept as a liquidity reserve for unexpected expenses) does not speak for a top interest rate.

How much of the [Grundstücksdarlehen] has already been repaid or how much equity is in it? ( = [Grundstückswert] minus remaining loan)

It may be that I expressed myself somewhat unclearly. The loan for the land is a flexible loan. The financing of the single-family house is to be carried out by the same bank, so they would repay the loan for the land and a total amount would be financed. The decision was already made when purchasing the land. Thus, the approximately €300,000 is the total amount including land, house, and all incidental costs. I will arrange a new appointment with my advisor in the coming days and see what he has to say, let's see if it can work out or not....

The financing could work with that income.

However, I would question again whether everything is really included at that price. I don’t think that all incidental costs (e.g., also land charge registration with the notary and land registry) and the furnishings (kitchen, furniture, lamps) and other things like painting work, floor coverings, and the outdoor area are included.
Take a look at a few cost breakdowns here in the forum and write down all the items. Then go to your provider and ask what is really included and recalculate the rest. A good calculation with a sufficient buffer is the basis for the financing.

Land registry entry, notary, and property transfer tax have already been paid with the land purchase. We have included all incidental costs according to the internet and supplementary costs from the house provider. Here is the current breakdown of how these are composed.

Painting and wallpapering work €4000
Floor coverings €4000
Water connection €2400
Electricity connection €2500
Sewer connection €4300
Telephone €300
Terrace/paths/driveway €3000
Planting €2000
Miscellaneous €12500
Surveying €1200
Administration insurance €450
Building permit €850
Operating costs during construction €600
Construction completion cleaning €300
Construction WC/container €600
Miscellaneous €1500

If something major is missing, I would be grateful for additions....

So the Telekom connection costs 400 euros. What I meant are the costs for registering the land charge in the land register and with the notary. That also costs a pretty good chunk. What about furniture, kitchen and a buffer for unforeseen expenses or additional equipment?

So the connection from Telekom costs 400 euros.

What I meant are the costs for registering the land charge in the land register and at the notary. That also costs a good chunk.

What about furniture, kitchen, and a buffer for unforeseen expenses or additional equipment?

We will take most of the furniture with us and complement the existing pristine kitchen. I would have to recalculate exactly what costs arise for the land registry entry. The rest of the equipment is included in the €12,500 miscellaneous.

I had only read the lower 1500 euros miscellaneous. ;)

You should calculate the land charge between 1500 and 2000 euros, depending on the amount. But there are calculators on the Internet.