Now there are the following questions:
1) Of course, we would prefer to keep the houses. How do banks usually view rental income (we live in a popular student city)?
The bank checks whether the rental income is plausible and therefore sustainably achievable. This depends on the size and the local customary rent. If this is the case, your income will be credited, taking into account a deduction (as already indicated here – simply because of management costs, etc.).
Can we, for example, set aside €400 of the rental income and plan €1000 for the installment?
How you ultimately plan/spend your money is relatively unimportant to the bank. The bank calculates, very simply put, income (earnings, rent, etc.) minus expenses (household allowance, additional fixed costs, loan liabilities). If the sum that remains as surplus at the end is sufficiently high to meet your intended loan obligations (also taking into account factors such as interest rate change risk, retirement, etc.), there is no creditworthiness obstacle.
2) Is it perhaps smarter to sell one of the houses if you know that, in the long term, you will acquire more properties (parents’ house, girlfriend is an only child)?
Counter question: Can you have too many (paid-off) properties? From the way you describe your financial situation, I see no reason for this, unless you feel uncomfortable with the higher installment compared to the rate you would have if you sold one house and consequently contributed more equity.
3) Which monthly installment would you consider reasonable with our income/expenses? €1300, €1500, €1700?
That is always difficult to say in general – or actually impossible. The question is what you WANT to afford. Purely in terms of ability, neither €1300, nor €1500, nor €1700 seem problematic to me. I also see the topic of residual debt at interest rate change (depending on the fixed interest period) as rather uncritical for you, since there is sufficient substance.