Any advice on a gap year in Japan?

A lot of this can be found online but your international office should also be able to help. If this is offered as part of your degree there will be someone who helps with this as well and who may already have all this info.

alleycat is absolutely right! But since I have a lot to share about this topic, I'll still try to answer your questions. (Just my personal knowledge, I might not be right in every single detail!)

I'll be going to Japan in January and stay there until July as a student, learning Japanese at a language school in Toyko. Therefore I already went through all of this!

1. Basically, a work & holiday visa and a student visa are sufficient to work there part time, work visas are quite hard to obtain anyway. Both have a few requirements, for a student visa you obviously have to attend academic institution in Japan, a work & holiday visa has some limits in terms of age, etc. (http://www.uk.emb-japan.go.jp/en/visa/working-holiday.html) Still, the work & holiday visa is mainly meant for travelling, so there might be a few limitations there!
2. As mentioned above, those two visas in genereal allow part-time working, but there could be a few limitations concerning the job, salry, etc.
3. The Student Visa at least has a few fees which have to be paid to the school, the immigration office and the embassy. (I think around 40.000+ Yen)
4. Absolutely no idea.
5. I will be part of national health insurance, which is completely paid by my school and the governemnt. (This is at least the case for student visa!)
6. Just a start: http://gogonihon.com/en
7. For me it was mainly the experience that motivated me to do this! If you just want to improve on certain skills and work at the same time, I do not know if it might be the wrong choice, but that is a very personal thing!

Hope I could help a little bit

Can you read and write Japanese as well? Speaking it is just one-third of the language equasion. The general world of Japanese business is in no way prepared to deal with someone who needs to work with documentary inputs/outputs in English. It really isn't a culture which encourages non-Japanese workers - a friend of mine lived and worked there for fifteen years.

It may be worth looking into study-based experience rather than seeking something which is purely an industrial placement.

Not trying to be annoying (well...), but I am going to reply in Japanese (I'm native)
全部の答えを知っているわけではないけど、運転免許は郵便局（ここの）で国際免許証を申請すればそんなに手間もかからず取得できるはず。18歳以上だったっけな？
あと、通常の旅行者のビザは９０日間で、就労は難しかった気がするから、よく調べたほうがいい。

http://www.studyjapan.go.jp/en/inj/inj03e.html
このウェブサイトで説明してるし、ほかのところも見てください。

あんまり役に立たずごめんなさい。
自分も諸事情でいろいろ調べているので、なにかいい情報を仕入れたらここに戻ってきてお知らせします。

oh god my Japanese post really didn't work did it?

Ahh that's fine, don't worry about it

そうなんだ？それは良かった！そう、２１歳は大丈夫だよね？
うん、なんか普通の旅行者が仕事をできないみたい。

これいいね！役に立つかもしれない、ありがとう

いえいえ、本当にありがとうございました。助かりました。
了解です！