is there a way to schedule a turn-on-wifi-sync-turn-off-wifi scheduled task for every day at 8am? I have no 3G.
I use my android phone mostly via wifi home network and read emails and podcasts later offline.

Is there a way to make a scheduled task which will turn on wifi, force sync (or wait for it) and turn off wifi after for example after 20 min?

Force sync is optional. I will just hope that during the 20 min it will occur somehow.
Is there an app which will do this or some smart hack?
Of course there are tons of it. Many of them just supports what you are asking but I'd recommend you llama or tasker. Tasker is paid app but llama is free and uses your cell tower to determine your location to make location based tasks as well. For eg, not only it can do what you have asked but also only when you are at your home.