1. How realistic is our plan to take out a loan for the land purchase? Does the fact that I already have an employment contract for March (of course, probation period...) play any role at all? Is there a bank that you would especially recommend?
Currently, you have no income, and normally that is not taken into account. There is no guarantee that you will start the job or not be terminated after 3 weeks.
2. What should we consider with a two-part execution (Loan 1 for €340,000, Loan 2 then for approx. €280,000 by using the equity from the apartment sale)?
Very important: do not take out loans with fixed interest rates but finance the land only with a variable rate. Otherwise, one bank will hold the first priority lien in the land register, and no other bank will want to grant a second loan subordinated. The conditions will then reflect the lack of competition.
With variable financing, you can terminate the first loan and combine both into one large loan.
I have also considered whether only KFW would be an option for the second loan (2x €120,000 from the Efficiency House 40 program – we are still planning a rental apartment – and the remaining €40,000 might also come together somehow, or for that we might still have to take out another loan). Is it conceivable that a private bank as well as the KFW would go along with such a model?
There are banks that also mediate promotional loans without their own tranche. My financing through a savings bank consists of 1 KFW and two L-Bank loans. However, the point remains that the mediating bank wants to be entered in the land register, and this is only possible if no other bank is listed there. But it’s an interesting approach to refinance at one bank. What I see as critical is that after the first longer term, you are at the mercy of the bank.
3. My brother-in-law said you can also take a loan without special repayment options and thus with a slightly lower interest rate, because thanks to the special termination right you can just put the money aside and then "pour it in" after 10 years as much as possible and also continue to make special repayments without the interest agreement made for e.g. 20 years being canceled.
Your brother-in-law is wrong there. You have the right to terminate the entire loan contract after 10 years and settle it with money you get from somewhere. Changing conditions unilaterally is not allowed.
I am with the others on this. First clarify what is going on with the land. Something does not seem quite right there.
And then get the land with bridge financing and properly finance according to the construction contract.