Buy a Reihenmittelhaus KfW 55 standard and rent it out later

I think so too... first acquire and live in it. In 7 years, the following scenarios may then arise:
1) No need for action. You like your RH very much by now.
2) Build: Interest rates are quite high now. Sell to borrow little.
3) Build, interest rates still low: Borrow a lot and continue to use the RH as a capital investment and rent it out. -Attention: Consult a tax advisor.
Karsten

Household income is just under 5000 euros net per month. (I am 25 and my girlfriend is 23). My expectations are 5% repayment; for this property, that would cost us just under 1500 euros per month. After moving out, we would reduce the repayment to 1% to finance our new property. We are really a bit scared to take this step because of course very high debts will come upon us, but the terraced house in the middle should eventually pay for itself through the tenant? Should we invest all of our equity or just the incidental costs like property transfer tax? Equity amounts to 70,000 euros.

You must agree on the repayment rate change very precisely if it is important to you. There are many different arrangements. Some contracts do not allow it at all, others allow it 1-2 times free of charge during the contract term, and others once a year free of charge. The range of repayment rates also varies; in my experience, hardly anyone offers a reduction below 2%, many have specific corridors, and some do not go below the initially agreed repayment rate.

I would invest all equity completely unless you plan to invest it elsewhere. This does not mean overnight money, fixed deposits, etc. Every additional euro of equity can improve your loan conditions or generate returns in the form of saved interest and that tax-free. You will need the 70,000 euros anyway. Purchase incidental costs (20-25k euros) gone, kitchen (10k euros) gone. Are flooring and painting already included? That can also cost a lot. Possibly adjust outdoor facilities; developer properties are usually finished outside but very minimalist. What about furniture, other than the kitchen? Lamps? Lawn mower, patio furniture?

Whether you really want to rent it out... that can wait. Possibly it makes sense to live in the property for 10 years or at least hold it (fixed-term rental contract) and then sell it tax-free. With a purchase price of 255k euros, 10k euros for the kitchen, and 25k euros for incidental purchase costs = total 290k euros, the rent would have to be 1200 euros netto per month to achieve a 5% return. Deducting maintenance, interest, taxes, etc., there is not much left to justify the effort. But some people just like that.

Thanks a lot in advance for all the good opinions! I only see a problem if I were to sell this property from 10 years ago, unfortunately capital gains tax would be due and that is rather unpleasant. For us, it is definitely out of the question to live in this house forever because we want to move closer to my workplace. As I said, this should only be a temporary solution! I just find it extremely difficult to decide, paying rent is really annoying, but you don’t have to worry about anything because the landlord is responsible. Buying a house means all costs are on me and renting out, I believe everyone knows you can really get into a lot of trouble with tenants. On the other hand, I see buying as a way to invest your capital profitably, you get nothing from the bank at all and I buy stocks every year through my company and that is enough for me. So where to put the money? The biggest problem is if we find a plot of land after 2 years, will we still get money from the bank?

Yes, there are then interim financings if you sell the "old" house. If you want to keep it, then I don't know. Was the sale not tax-free after four years for owner-occupied property?

I had understood so far that you will not buy the "real" house anyway in the next 7 years. But if you want to have the option to find, buy and build on the dream plot in 2 years, I would position myself as flexibly as possible. That means staying a tenant, increasing equity and keeping the capital available. Selling the townhouse after two years would be bitter. Purchase incidental costs gone, price loss because no longer new, ...

If we get our dream plot, we would want to buy it immediately. That's why it's a bit of a gamble, either we are lucky or not. We are on a list for a building area at position 23; if the nice plots are gone, of course we would have to keep looking. If we find our dream plot there, we could of course extend the construction start date a bit, buy the plot and start pouring the slab after 2 years and then wait again. But is that the right thing to do? Will we get money from the bank again after such a short time? The house will certainly cost >350,000 euros.