NoSQL DB For .Net Document-based Database (ECM)?

I'm halfway through coding a basic multi-tenant SaaS ECM solution.Each client has it's own instance of the database / datastore, but the .Net app is single instance.The documents are pretty much read only (i.e. an image archive of tiffs or PDFs)I've used MSSQL so far,but then started thinking this might be viable in a NoSQL DB (e.g. MongoDB, CouchDB).The basic premise is that it stores documents, each with their own particular indexes. Each tenant can have multiple document types.

e.g. One tenant might have an invoice type,which has Customer ID, Invoice Number and Invoice Date.Another tenant might have an application form, which has Member Number, Application Number, Member Name, and Application Date.So far I've used the old method which Sharepoint (used?) to use, and created a document table which has int_field_1, int_field_2, date_field_1, date_field_2, etc.Then, I've got a "mapping" table which stores the customer specific index name,and the database field that will map to.I've avoided the key-value pair model in the DB due to volume of documents.

This way, we can support multiple document types in the one table,and get reasonably high performance out of it,and allow for custom document type searches (i.e. user selects a document type, then they're presented with a list of search fields).

However,a NoSQL DB might make this a lot simpler,as I don't need to worry about denormalizing the document.However,I've just got concerns about the rest of the data around a document.We store an "action history" against the document.This tracks views,whether someone emails the document from within the system,and other "future" functionality (e.g. faxing).

We have control over the document load process,so we can manipulate the data however it needs to be to get it in the document store (e.g. assign unique IDs). Users will not be adding in their own documents,so we shouldn't need to worry about ACID compliance,as the documents are relatively static.

So,my questions I guess :Is a NoSQL DB a good fit Is MongoDB the best for Asp.Net (I saw Raven and Velocity,but they're still kinda beta)Can I store a key for each document,and then store the action history in a MSSQL DB with this key?I don't need to do joins,it would be if a person clicks "View History" against a document. How would performance compare between the two (NoSQL DB vs denormalized "document" table)Volumes would be up to 200,000 new documents per month for a single tenant. My current scaling plan with the SQL DB involves moving the SQL DB into a cluster when certain thresholds are reached,and then reviewing partitioning and indexing structures.