600W is good for that system, putting an inferior quality supply in any system does increase the chances it will die/kill some other components when it does. I always say do what you feel, so if you feel good about it, go for it.
Any advice on a good quality alternative for around 50-65 euro?
im currently working, but if you want to do some research, search for PSU tier list on google, there should be a link to newegg forums where there is a PSU tier list.
How would a Seasonic S12II 520W run things if I for instance upgraded my graphics to something like the HD 7850?
That would be suitable, seasonic's a rock solid brand too so if you have any problems they should step up to the plate.