For everything except your first requirement it doesn't really matter. The Japanese IME is in any language version, English versions of Office and Visual Studio run on any language version. I can't imagine using Japanese drivers on English Windows is going
to be a problem (you might have trouble running the installer if your system code page is not set to Japanese, but you can change that regardless of the UI language and there's also AppLocale).
Your first requirement, having different UI languages for different user accounts, can only be achieved with Ultimate (and Enterprise, but individuals can't buy that). At least that was the case with Vista, I think it will be the same for 7.
My recommendation would be to get Windows 7 Ultimate in English, and then install the Japanese language pack from Windows Update. This will allow you to set up one account in English and another in Japanese. Or buy it in Japanese and install the English
language pack from Windows Update, it makes little difference.
Don't mind Koogle, he makes little sense to anyone except himself.
I can't imagine using Japanese drivers on English Windows is going to be a problem
I can. Localized versions of Windows have a localized version of the Registry, if the drivers are dependent upon things in there it's quite possible they will fail. This is different to MUI versions, which have the Registry in English and only the UI gets