The classic GDR folder must be the old 'MIFA Klapprad'. Some nice examples with a lot of charm to be had, although I doubt it was 'really good' from any design or technical viewpoint.
If it's just a bike for guests you're after and you don't mind it being newer and less exciting, the folders sold by Aldi unter the 'Curtis' name are robust little bikes with lots of alloy parts and Shimano hub gears. These turn up second-hand fairly frequently and shouldn't cost more than EUR 100.
Last edited by Elvis Shumaker; 05-20-14 at 01:27 AM.