There is an advantage in them being used, as a Dahon made specifically for a market with low incomes would be not be in a shape to be resold after a short while. From these I would likely go for the first one, as it has a dynohub, dynamo lights, Marathon tires and Sram shifter. It looks like the owner was caring for that bike and upgrading components. Some concern is in the rust, but at this stage it still looks superficial. With folding bikes of concern are hinges, whether they are in still good shape. I would fold and unfold the bike, ride it around, looking for any feeling of insecurity around the hinges. Look inside the frame whether the rust is just slight on the surface or are there more serious concerns. Pricewise around 200 euro for used steel folding bikes is about what you should pay - maybe you could push lower for the other ones. As to the rust,I would spray the rusted areas with ACF-50 or another similar motorcycle treatment.