Alex, assuming you are attending a buddist wake, needing a rosary or not depends on the custom which differs mainly according to the region. For example, in greater Tokyo, it is quite common for hardly anyone to be carrying a rosary, but in conservative regions, everyone is expected to hold one when greeting the deceased.It would be best if you can ask someone attending the same ceremony. If not, ask a rosary sales clerk (such as one in a department store) for advise. If you tend to live in Japan for several more years, it might be a good opportunity to buy an appropriate rosary fit for your gender and occasion.
The amount of cash also differs depending on your relationship with the deceased and the custom of the region. Best, go to a stationary shop or department store to buy the envelope and ask for advise there. The bills shouldn't be fresh, by the way. They should be fairly neat but used.
That said, non-Japanese are excused of any cultural mistakes they may make.
