I'd like to answer your questions point by point.
>Are the Japanese universities very tough? Are they expensive?
It is difficult to solve your question because I don't know how much is school fee average in your country. However, you will find many scholarship applications foreign students if you enter Japanese university. Moreover, you should choose public school and can keep down the school fee.
> Do students have to work a lot to pay for their studies? Is there still some free time during evenings or weekends - or is it just study and work until you graduate?
No, they maybe have free time on weekends. I have a friend who are a foreign student. He seems not to work part-time at all. However, your question depends on your major. If you want to study in scientific Lab, you cannot have more free time than humanities course.
>What is it like to work in Japan? Start early in the morning and finish late in the evening?
Umm, it depends on what job you choose. Japanese work used to be said an overworker, but recently old labor condition is improved. The average working hours per a day is 9 hours now, so if you come to work at 9:00, you can quit at about 19:00. Whether its work time is long or not is up to you.
>Is it difficult to get a job? Can a European get a job in Japan if they have an education and good knowledge of Japanese (N2-N1)?
I think you don't have to worry about it. It is said that Japanese are not better at English than other countries. So, many Japanse companies want to employ foreign students, I think.
I'm not from Tokyo or Yokohama, but if you enter Japanese school, I'll tell you about there.
I hope this helps.