If it's just for external display purposes, you don't need a dock - just connect the monitor to the system with an HDMI cable. If you want the dock to charge the system, you'll probably want a Thunderbolt dock with at least a 65W power output -- 90W if your system has a discrete GPU option. The Dell WD22TB4 will do the job.  So will some less expensive docks but you'll need to choose based on what you plan to connect.  
I am trying to use my Dell P2319H monitor with my Dell Latitude 7320 laptop.  Can someone recommend the best docking station?  Thanks
Docking station for monitor and laptop