You don't need a second 5K screen and besides no one has an external 5K display. Get one of the Apple Thunderbolt 27" displays while the supply lasts. You will also need more storage. I think you can get a 1TB SSD. But you will also need an external RAID. I would recommend Promise Pegasus.
Just what I have, with a Pegasus R6 and two WD 4TB USB3 drives for Time Machines. Perfect combo.
Word of warning: you will plug in the 27" Thunderbolt display, look at it and think "You are beautiful". Then you will look at the 5k iMac's display, and then look back at the Thunderbolt display and wonder what you ever saw in it :-)