Appropriate style and attitude depends on the situation.
I hope if my opinion will be a little help for you.
Having tatto is not impolite matter therefore you don't need to be seriously. However some peoples who have old (traditional) thinking may be embarrassed if the tattos are scary images.
2) financial assistance
It is usual that the host persons take a hotel room, and pay for whole (or a part of) transportation fee for the guests coming from a long distance. This financial assistance means that the cost to participate in the ceremony. I guess that two or three nights hotel charge is reasonable price which she will pay. You can, of course talk to her about this matter.
3) gift of money
Right, it is still teh custom. The money gift has two meanings. One is the feeling of congratulation, anf the other is for preparation fee for the ceremony.
How much we give as gift for depends on the status and relationship. 30,000 yen is reasonable and enough amount I guess.
No other gifts will be needed in the ceremony, however we often prepare small gifts when we visit the host in different opportunity.
You don't need to bring a gift for the family members in the meaning of wedding gift, however they must be glad if you bring some souvenirs from foreign country for them.
If the guests will pay participation fee in the ceremony, you can only pay for the fee. In addition it is smart and polite if you will bring a small gift such as flowers.
4) suitable dress
It also depends on the ceremony style, time (day time or at night) in strict.
However you don't have to be sensitive, please just care for the points below;
* White color is only the color for the bride. Please avoid a white color (or light silver) dress. On the other hand, black color dress will offer dark impression (men's suit is OK with gorgeous tie). If your dress color is dark, you can add gorgeous accessaries.
* Please ware stockings. Stockingless legs are impolite. The type is prefered to be gorgeous one such as having lame, however please avoid net type. To ware a boots is also impolite.
* If your dress is shoulder opening type, you had better to ware a shawl or a bolero to cover your shoulders. It is also polite.
This web site is in Japanese, however you can imagine the popular style.http://www.rapty.com/party/style/
hope to have fun