There is no dock that I know of that will also make your iPad a second monitor. A dock and a secondary display are two different things.
There are several apps in the App Store that allow you to set the iPad up as a second display for your Windows PC. I use an app called DisplayLink, which is free. It does require you to install a driver on your PC.
This is not an endorsement of this app, mind you. I'm sure there are more in the App Store that work just as well as the one I have. YMMV.
Hope that clarifies and helps. Good luck and happy searching.
Marilyn