Is home financing with mother possible?

€280K + purchase incidental costs (assumed 12%) minus equity = €289K financing
Generous 1.5% interest and approx. 5% repayment (for a 20-year term) then results in a monthly rate of €1,570.
This should - without further information about the personal situation, the condition of the property, etc. - be acceptable to a bank. More input is needed for a more informed statement.

Or he is married, but separated and is a single parent.

According to various calculators on the internet, I come up with a rate of 800-1000€ per month.

Is the amount of 280,000€ set too high (as the maximum possible amount for the house)?

With which data? Keep in mind that your mother should not finalize her financing with only 2% repayment and a notional term of 35 years or something similar.

I don’t know where you live, but €280,000 for a house seems incredibly cheap to me. Especially for a two-family house.