Separation of UI and Camera Viewport

Hi folks.
I need to organize my game interface as you can see from the image:

So I need to have my camera always centered on some object in resizable area, and I need my interface be out of camera viewport.

Is it possible?

I know there is a Render Targets, but I have some Lighting and Post Effect problems when I use it. And before I will move further to that direction I need to know:

  • Is it the only way to do what I want?

  • How heavy it is in comparison to ordinary camera rendering?

It is ok to achieve only this separation of camera and interface, but ideally I’d like to have two camera pointing to different objects like split-screen.