Task/File/Project Manager (Basecamp Clone)

I am somewhat experienced with Drupal, and recently I have found a need for a project manager in which I can allocate tasks, and collaborate with others in an organisation.

I need to build a BaseCamp Clone, with tasks (within milestones or 'subprojects), notice boards, files, calendar, and project timeline.
All of these items must fall under each 'project' heading, with the site being able to support multiple projects.

I am not very experienced with PHP, so I would rather not struggle modifying modules, but if there are small modifications that would make my life easier, that would be great.

I was thinking about using OG as the basis, having each project as a 'group' then building the rest of the structure under that, however I am not sure how to implement the calendar, tasks(with milestones) and I am not sure if any calendar modules can operate within the OG structure, only revealing events to those who are assigned members of the project.

Any suggestions are welcomed.
Thanks all,
Michael Wheatland