New Large Scale Project.

I'm looking to do a fairly large scale project. The problem is I don't know how much of it to write myself how much I could get away with using existing components (modified as necessary). Basically the different areas I would need are authentication, galleries (for images), permission levels. Internal messaging (not real time), forums (hopefully I can just plugin an existing one but intregrated with my new so that registering users for the site registers for the forum and vice versa, also I would like each new gallery entry to create a new forum entry for that image). Comments, ratings and such like. Sales (This would have to be a script written elsewhere, I am in no way confident when it comes to dealing with sensitve info like credit card details.) etc. etc. My other problem is even though I'm confident coding I find design very difficult I've a good idea of how the website would work but only in general terms. Out of the box mambo or drupal doesn't seem to do what I want really (obviously) but maybe they would give a good foundation in which to plugin my own building blocks, has anyone tried this? Regards, Wes