I have a .NET 4.0/WPF based application I've developed and maintain for my company that acts as a friendly GUI central-point-of-information, combining information pulled from a couple of SQL databases, as well as CSV exports from a few other applications.

I would like to build out my own database to support the entirety of the information that the application accesses, so that I could have a service running on my server that would read in necessary remote SQL info and file exports, to provide the user's application with a single database to connect to, as well as to remove all of the file handling currently involved in the program (copying new CSV resources from network location, reading them into memory each launch.)

I have complete control and flexibility here as long as the user's experience isn't affected, and this is as much a learning experience as it is tidying up. Caveat being, I don't have much in the way of a budget.

Right now I recognize my options to be:

• SQL Express - I'm comfortable with the server setup, I like ADO.NET and LINQ to SQL. I feel that I have the least to learn here, but it would let me focus on SQL in a familiar environment. Perhaps in conjunction with Entity Framework?
• MongoDB - I don't know a whole lot about, but I've heard the name enough to make me curious. Brief research seems friendly enough, and there is .NET support. I like working with open source projects.

My questions are:

• What's popular and extensible right now? I'm not far from starting to job-hunt, and I'd like this project to be relevant going forward.
• What am I missing? Pros, cons? Other options? What plays well with .NET?
• What are the things I should be considering, the questions I should be asking, when making a decision like this?

Thanks for your time.