Is is possible to combine top panel bar and dock? My best option would be that combined bar to show on the top, always visible.
As far I know, no. You can change the position of this dock using
elementary-tweaks, but if you move it on the top, it will go under the top panel which is not really convenient, because you barely can click items.
The fact that Plank comes with the option to be shown at the top acts as a bug in the context of the elementary desktop, as it interferes with the panel (Wingpanel) no matter what. The question amounts to how to reduce the panel length (left-right size) in order to leave some space for the dock at the top. They cannot be combined.
I don't think that reducing the panel length is possible, but I have posted a specific question on that in order to have a clear answer.
On the other hand, for what it's worth, as this is a matter of gaining display space: the elementary desktop has its own way of taking advantage of display space through the use of shortcuts that may remove the need for what we ask here. Super-A and Super-S, along with Alt-Tab and Super-Tab allow access to windows and workspaces.
Adding a shortcut like Super-F to "Toggle Fullscreen" may also be a good idea. The fullscreen mode assigns to each window its own workspace, which can be cycled with Super-Tab.
That might also reduce the need for the dock to be always visible, hence more display space.