About SharePoint:
- There is a permission management in SharePoint. You can set user permissions up to individual items, files and pages. You can read about it here: User permissions and permission levels in SharePoint 2013 and here: Fine-grained permission reference for SharePoint Server 2013. You can use differend authentication providers (AD, FBA, Oauth etc): Authentication, authorization, and security in SharePoint 2013.
- You can create customization declaratively (no code), С# or javascript. There is built in object model (client and server) and web services (REST for 2013 or SOAP for 2010). More about SharePoint for developers
- You can use built in form web parts or create custom with server code (C#) or with just HTML and javascript. All you can do with asp.net - you can do in SharePoint with custom web parts. But you can do most of things just with HTML and javascript.
- You can request web services from server code (C#) or from javascript - just like in asp.net apps.
- You can do db search with javascript or server code, using your prefer datagrid controls.
I think most easy way is using SharePoint 2013 Foundation (it's free if you have Windows Server license). And using javascript and HTML - in this way you don't need to know anything about deploying solutions in SharePoint etc. But you can do it via custom web parts - in this way you may do it like common asp.net development. But it's not a best way.