Unfortunately, nationality does matter here. If you try to come into Japan with a one-way ticket and no visa, they will likely prevent you from boarding the plane. I was stopped and I HAVE a visa.
As mentioned, do you have dual nationality? You mentioned having both a British and a German passport. With either, you will be eligible for a working holiday visa if you turn 18 before you leave. Your father will not be, unless he is under 30 years of age.
http://www.uk.emb-japan.go.jp/en/visa/working-holiday.htmlSome companies require a Japanese bank account and most (if not all) require you to be in Japan to sign the contract.
It's not that you can't stay, but you need to ensure you have all the necessary paperwork done. While it's not impossible to receive a more permanent visa while in Japan, it's notoriously difficult. My friend BARELY managed to get all the paperwork done before her 90 visa expired.
For a PERMANENT visa, you have to work in a contributing job (not manual labor) for usually 5 years or more.
As for food, entertainment, etc...I live rather frugally and live on between 5000 and 10,000 yen per week, depending on what I eat that week. If you want to enjoy yourself, budget at LEAST 50,000 yen per month.
Not to be rude, but as mentioned several times, we're here to help you but if you act like that in Japan, you're going to run into problems. This forum gets a lot of questions just like yours (but usually without so much attitude). In fact, a simple Google search might have answered most of yours...