You never mentioned a passport that is self contained. They will not install in a enclosure.
But 2.5 wd reds will need one which usually have a power source to them.
The wd reds are cheaper but require a enclosure which cost a little more.
Hard call you need to compare passports cost to wd reds plus enclosure.
Probably a wash.
the wd red option will put less load on the server psu due to it's own power supply plus you will still have 3 usb ports free.
The passports should work but I can't find specs on the power limits of each usb port. Plus you will take all the usb ports unless you buy a usb powered hub. So if you need to expand again then what to do with the passports other than buy a usb hub to free up usb ports. The hub is self powered.
Powered usb hubs are cheaper than enclosures so again price shopping is if you want to go the passport route.
For example I see a 4port hub for like $18 at amazon
https://www.amazon.com/RSHTECH-Splitter ... +hub&psc=1A 4 port enclosure at same place $54.99
https://www.amazon.com/Sabrent-drive-Do ... B0711L68MSJust a quick look