Land financing, variable loan?

Thank you for the answers, that's roughly what I expected.

The plot has no building obligation. I will only find out the exact price later; I was told at the beginning of the year to get in touch in the middle of the year, as it is currently not being sold (I guess for tax reasons). €125/sqm was the previous price, but I was told it will not stay that way and will be more expensive. For the location, €125 is already set quite high, so I simply assumed €150 and came to around €100k.

We do not want children; this has already been medically ensured.
I have done a lot of calculations over the past few years and we were already at a bank just before the price explosion. Since then, a lot has happened.
It's hard to say exactly what the income is because I work shifts. I earn about €3400 net (maybe €3500, I can determine more precisely next January), with special payments already included. I am a member of IG Metall. My wife currently earns €2000 net and will receive another increase in a few months estimated at €200-300 net. Together roughly €5700 net. No major increases are expected except for the usual collective bargaining adjustment/inflation compensation.

If necessary, I could resell the plot if it doesn't work out with a house. The transaction should not cause much loss, if any. The house should also only be a small bungalow without much fuss, about 100-110 sqm. But even with that, I come to a total rate of about €2500/month +-. I have to admit I haven't checked house prices for at least half a year since the main factor is the interest rates, which have remained roughly at around 4%.

As soon as I have spoken to the seller about availability and price, I wanted to make an appointment with a bank. I didn't know there were fixed loans for 3 or 4 years; I assumed a minimum of 5 years.

I have done a lot of calculations in recent years, and just before the price explosion, we were already at a bank. Since then, only a few things have happened.
Income is hard to specify exactly since I work shifts. I earn about €3400 net (maybe €3500, I can determine more precisely next January), with special payments already included. I am a member of IG Metall. My wife currently earns €2000 net, and in a few months, she will get an increase of an estimated €200-€300 net. Together roughly €5700 net.

That is a good foundation as a couple.

The plot has no building obligation. I will only find out the exact price later; at the beginning of the year, I was told to get in touch mid-year since it is not currently being sold (I guess for tax reasons). €125/sqm was the previous price, but they won't stay that way; I was told it will be more expensive. For the location, €125 is already considered high, so I just assumed €150 and would arrive at about €100k.

So (a reasonable) option: buy the plot, practice restraint for three years, and save €3000 every month in that time – repayment for the plot + savings rate.
Then, if nothing extraordinary happens, in three years a

small bungalow will be about 100-110 sqm without much frills.

affordable.

Let the bungalow then cost €300k, which you have to finance. That would currently be a €1250 installment, and the house would be paid off before retirement.

But even with that, I come to a total rate of €2500/month +-.

What are you calculating?

Sounds good.

In an old rough calculation, I had noted this:
House: 300k€
Additional costs: 50k€
Garden: 10000
Carport/Garage: 10000
Paving: 10000
Kitchen: 10000
Miscellaneous/Furniture: 5000
Buffer: 5000
Solar: ?
Total house: 400k€
Land: ~100k€

If I now use the forum calculator here, I get a rate of about 2500€ monthly for 25 years at 475k€.
But I think some of the costs above are a bit underestimated, garage, kitchen for example.

Sounds good.

In an old rough calculation, I had noted this:
House: €300k
Additional costs: €50k
Garden: 10,000
Carport/Garage: 10,000
Paving: 10,000
Kitchen: 10,000
Miscellaneous/Furniture: 5,000
Buffer: 5,000
Solar: ?
Total house: €400k
Land: ~€100k

If I now use the forum calculator here, I get a rate of about €2,500 monthly over 25 years at €475k.
But I think some of the costs above are a bit underestimated, garage, kitchen for example.

The house is okay, it can also be a bit cheaper without any demands.
Additional costs are also somewhat lower with a flat plot and a standard house.
Garden: Own work, much cheaper.
Carport/Driveway etc.: can be done immediately or later. At least fresh air is good for the car.
Kitchen, furniture etc. have nothing to do with house construction and are not financed.

If you have the land as equity in three years and also save up for the kitchen and furniture, only the bare house construction remains.
Let it really be €400k that you need. It currently costs about €1,700 per month to be debt-free before retirement. Of course, you can also torment yourself and pay €2,500 per month. But it is not necessary.

Oh yes, even €475k can be stretched with 1.5% repayment. Maybe you can make a special repayment in between. You both earn quite well. Then the rate is +/- €2,100.

It says Garten 10,000 not 100,000 EUR. That is even with own labor the absolute minimum that you need for tools and materials even with 100% own labor. Renting a vibrating plate for 2 days, 150 EUR. Mini excavator per day 120 EUR. And woe betide if L-stones need to be set. Then 10,000 EUR is the down payment.

It says garden 10,000 not 100,000 EUR.

Yep. Garden. Not paving (has its own item), not L-stones. Just garden.

Rent a vibrating plate for 2 days, 150 EUR. Mini-excavator per day 120 EUR.

Are you flying them in from Spain? Vibrating plate from 30€ per day, for 2 days I get the fat 600kg vibrating plate for 150€. The 1.8 ton mini-excavator with bucket, trench work or similar (although "mini" is relative here) should be around 60-80€. Don’t make everything so bad and expensive. If anything, you just let the local earthworks guy do the rough work for a crate of beer and a few browns.