Correct database solution?
My initial reaction is that you should have everything you need in a standard Microsoft SQL Server installation. Integration Services to do the ETL from your flat files into SQL DB engine. Then the T-SQL language and for your calculations and operations, .NET for more advanced operations. Analysis Services and Report Server engines for the online analysis/drilldowns and for producing reports for clients and staff. Couple that with MS Office, Excel and PowerPivot, and you'll have a complete suite of Business Intelligence tools for all your needs and more.

Just my 2c

Good luck!