If I read your message correct, Ziga, your doing 1100 miles per year, that is about 21 miles per week. Have I read that correct?
If so, unless you have lots of money you just want to spend why are you thinking of leasing a gen4 at €17000-€20000, its a nice car but a lot of money for something used 21 miles per week.
You could buy a nice gen3 for a more reasonable cost. In the UK the T-Spirit was the top model, but that may not get you leather seats - that must have been an option. You should be very happy whatever you get, but make sure it has had its service every, 12months/10000miles whichever comes first.
The gen3 you seen for €9800 (about £8800) would have to be from a dealer, preferably Toyota dealer, and in very good condition....... and haggle down the price, dont show your enthusiasm. If a dealer, go look at another car they got, then slowly work toward the Prius, saying your not too fussed, but asking eventually for a good price.