Your information and the offers from Interhyp just don’t add up at all.
You have a total volume of €425,000.
You want to borrow €310,000.
That results in a loan-to-value ratio of a good 73%, which will get you an interest rate of about 1.45% effective over 15 years if everything else is correct.
The interest rates from Sparda are 10-year terms, I’m quite sure about that.
Furthermore, you currently have a total net income of €1900, am I seeing that correctly?
The bank is not planning with your future part-time income.
That completely ruins the household budget calculation.
Depending on which bank you go to, the margin ranges from a few hundred euros to nothing per month.
Either your information is incorrect/incomplete/misunderstood by me, or the Interhyp guy really gave you very bad advice.