Yours is a tall order! I don't think any software exists as of now which can do all that you ask.
But as Roomy has pointed out, WindowsXP has support for all Indian languages. So on a WindowsXP platform using MSWord, you can work comfortably in Bangla.
But even this combination will not convert legacy fonts into Unicode for you.
For this you may resort to the TBIL Convertor, a freeware available for download from here:
This one can convert from several legacy fonts into Unicode.
Obviously it won't be able to handle all the legacy fonts in Bangla but only a few.