How do I develop a custom application with workflow

Hello everybody,

I am planning to develop a Sharepoint 2013 application that is tied to a workflow.

Here is what I want to do:

1. Start Workflow/Application when an item (Word-Document) is uploaded to a Document Library.

2. Analyze the document programmatically and show some options where he can add some information manually (in about 2-3 subsequential steps)

3. inform several people about the resulting information

4. End Application/Workflow

The points 1 and 3 seem to be still coverable by the usual workflow functionality. Unfortunately I cannot add some custom code to workflows as of Sharepoint version 2013.

Could you please give me a hint how to add custom functions and how to tie up the manual addition of information in an app or webpart (?) with the workflow that lies behind?

Thanks a lot in advance!