My biggest concern for you guys is how this will work from a management point of view. Any comments on that @JohnK ? If there's lots of contributions, someone needs to manage the codebase and make sure that feature trackers get updated.
From a technical point of view, that makes BitBucket + Jira an excellent choice given that they integrate well. Slack is also great for remote teams from what I hear.
For large assets, Git has lots of options. There's git-annex, which is a plugin which lets you store large files separately on a file server, but they can be checked out with the repository. Or new versions of Git have Large File Storage (LFS) built in, which I think does something similar. LFS has beta integration with GitHub but it's more expensive than just setting up an S3 bucket.