I'd like to use this combination with slr lenses for widefield photography, with ED80, and late perhaps with fast newton.
Is this combination any good for astrophotography? How long single exposures can I expect with it without trailing errors?
This "ADM" is actually the good old Celestron (Synta made) non-go-to CG5. Could you use it for astrophotography? Sure. The main problem with these is usually the large amound of backlash in the declination gear-system. Good balance, close polar alignment, short f/l, maybe a little gear adjustment and this could be fine for your purposes.