You should look closely at Photo Mechanic. It's not free or OSS, but it works very, very well. It is more of a metadata manager than a photo management database - it doesn't maintain its own database; it uses your existing folder structure.
It doesn't do any retouching, but it is flexible in letting you edit (select/reject), sort, and manage metadata for tons of photos. This sounds like the sort of software you're looking for.
It's practically the standard in the news/media photography industry, and it's widely used in other pro photographers' workflows.