Oculus Rift DK1 owner here, A few questions...

I’ve looked into using my DK1 rig for a bit and found the Oculus Rift runtime, but it seems geared for DK2. Is it compatible with both? What is the runtime exactly, just a plugin? Or is it an entirely different editor? Will I need to add a special VR node to my character in blueprints eventually? Thanks.

The runtime is an Oculus program which offers API’s to enable other applications like UE4 to communicate with the HMD. For the DK1 you should install runtime 0.6 (later versions are for the DK2).
This will work with 4.9 and 4.10, I’m not sure about 4.8.
Have the DK1 connected and switched on when you start UE4, in the preview dropdown you should see the *VR preview’ option enabled.
More info: Virtual Reality Development | Unreal Engine Documentation