Since you are in Germany, have you considered the German Riese & Muller Birdy? These come on ebay over there often enough. It may be attractive enough price-wise. It is considered one of the best folders out there, capable of going fast, but gearing range may be a problem for keeping up with the pack, unless you get one with a Sachs/SRAM dual drive.

Your budget limits you, unfortunately. It may be best to go for a fast Dahon like a Speed TT. Or a Dahon Mu P24, Jetstream XP. But these will require you to save up.