I would suggest using a more sophisticated hashing than simple check sums, to avoid the possibility of hash collisions. Possible hashing values would SHA-1, SHA-256, etc.
You can do this with a few lines of python on almost any platform by using the built-in libraries, specifically os.walk to traverse your directory structure & hashlib to compute the ...
Most grep tools can only search for a single regular expression. With PowerGREP you can use up to five sets of search terms. Each set can use an unlimited number of regular expressions.
Not free but neither is our time
If you are really serious about tagging, using taxonomies, and searching using them, then I would suggest a document management system such as Alfresco.
You can add metadata to any type of file, be it a PDF or a GIF or even some unknown binary format
Metadata can be of several types:
Tags: Lightweight, you can create new tags on the fly
I'd recommend you Tablacus Explorer
Another possible alternative is Free Commander. It offers a lot of features and even if you do not need them all, you can just turn them off from the Settings panel and then you'd have exactly what you are asking for. It is very customizable, so setting it up once would pay off in the long run.