I have a few questions (if I may hijack this thread) that I was hoping someone could address. Is the inability to speak in Japanese going to be a big problem? Also, I'm quite a fashion newbie. How helpful are the staff at the bigger dept. stores (where I'd probably end up shopping)?
Not speaking Japanese will be a bit of an obstacle. One problem I see is that people at the larger, very full service department stores such as Takashimaya and Men's Isetan, are more likely to be able to provide someone to you who speaks English. Unfortunately, the suits at those places are more likely to be expensive and outside of your budget.
I would recommend you visit the Suit Company. I believe that the largest location in Tokyo is in Ueno/Ameyoko, but there are locations in many many other neighborhoods. The staff at the Suit Company tend to be women in their late teens and early twenties. This means that they will be more likely to speak some English. The suits at the Suit Company are also more likely to be within your budget.