Thanks for your help!
You might want to brush up on your PHP skills, but you probably won't find it that difficult since you have experience developing for another php-based CMS. There's reasonably good documentation, and the devs are very responsive to forum posts. You might be able to gauge it better if you take a look at some of the code in one of the included themes--that's what you're most likely to be editing.
My concern, as an archivist, is that Omeka is designed for the presentation, not preservation of digital assets, and thus doesn't have any of the preservation features (like integrity checks) that you'd find in most applications intended to be used as a repository. So I'd make sure your clients aren't looking for that sort of capability before you get started.