Depending on the structure of the poster, in particular amount of text, use of external images, Scribus may be better suited but Inkscape could be your best tool for design of individual elements, while GIMP may also join the fray for some specific tasks.

From a study of your project in the marketing design phase (what the poster will look like), you will be able to think up the best workflow, and which tools will design which part of the final output.

Cedric