i am looking to answer one of those problems that sometimes get give to us devs by sales staff trying to get a sale in under budget.
We have a client that requires the following:
All up the budget for external software would be $30-40K.
This sounds like an impossible task as the budget for software and the timeline are a bit painful however that is the job and we need budget for extra dev and implimentation (not to mention the import) - which will be alot more than the cost of the software but not enough to pay for a custom solution that offers the same level of featureset and beauty
Sounds like a job for SharePoint. If you can get by with Windows SharePoint Services (which from the description you probably can), its a free part of Windows Server. It has document libraries for storing documents, great AD authentication, document upload features, the ability to add metadata to tag documents and build views for them, written in .Net, it scales well and a ton of information on the 'net about how to develop against it. Plus a whole bunch of features you probably don't need now but might be handy down the track.
SharePoint can be a beast but for your required feature list it looks like a good fit.
Although it uses SQL Server, the general warning with Sharepoint is to stay out of the database and do everything through the object model API or web services.