Advice on a monitor?

[PhoenixDown]

I currently have a 27" 2013 iMac w/ 5k Retina screen.
I also have a HP Elitebook G6 laptop for work.

Due to working at home, I am looking at buying (my $) a monitor for my HP laptop. I would also like to occasionally use it as a second screen for my iMac and eventually as a primary screen for a future MacBook Pro.

Any recommendations?

Screen quality wouldn't be too important for the work laptop but I can't see myself with a ****** display for my current iMac or a new MacBook Pro.

 

[DeltaMac]

If you have a 5k retina screen, you would maybe want to get close to the sharpness of that screen.
(I notice that you listed a 2013 iMac. A 2013 iMac does not have a 5k screen. Those were not available until the 2014 27-inch iMac. Maybe that's what you have!)
Something like a 27-inch, 4k screen should do you nicely.