I am currently also looking for good interior wall paint and came across "Schöner Wohnen Polarweiß" in the tests. The test rates the paint as very good. Here, 10 liters cost just €39.90.
Just drop by a painter's supply store near you. They have pretty much everything in stock (Sto, Caparol, ...) and also good store brands. In addition, you get 12.5L instead of 10L at a reasonable price (around €30-40 for the store brands). Most also mix shades and provide tips and tricks for painting...