A bit of background. Before the DK1 there were hmds, but they were expensive, didn’t have the latency nor crucially the FOV to be really immersive, that changed when a hobbyist, Palmer Luckey, showed off a prototype in early/mid 2012.
Based on the feedback, around August 2012 there was a kickstarter for what was to become the Oculus Rift (retrospectively referred to as the DK1), the kits started shipping in March 2013 and that kicked off indie vr development. Back then there was no UE4 so you could develop on UDK (UE3) and Unity.
Around Mar 2014 UE4 released, Playstation VR (then called Project Morpheus) was announced, HTC Vive was also announced, and shortly afterwards, in July 2014 the new development kit called Oculus DK2 started shipping. When people refer to developing VR games for UE4, this is most likely the unit they refer to. The DK2 introduced positional tracking, low persistence (removing motion blur from head turns), higher resolution as well as higher required FPS (75).
In December 2014, Samsung with the help of Oculus, released Gear VR, the first edition for the Samsung Note 4 phone. The Gear VR is a case which pairs with a compatible smartphone and turns it into an HMD. The Note 4 struggled with heat dissipation, but it still spawned the first mobile development for VR. Later, in March 2015 the second innovator edition was released for the Samsung S6/S6 Edge which had better thermal handling and GPU performance. Finally, in November 2015, the consumer Gear VR was released which had a slightly wider range of support for phones.
In May of 2015, Oculus announced that the consumer version of the rift, commonly referred to as CV1 will be released in Q1 2016. About a month ago, Oculus shifted production to the consumer version, and discontinued the DK2.
So where does that leave you and what are your options right now?
UE4 supports all of these VR options in some way. The engine has out of the box support of Oculus DK1/2/CV1, Gear VR, HTC Vive, Playstation VR (PS4 only), and OSVR support is currently available through a plugin. There is also google cardboard for simple VR.
The main deciding factor should be whether you want to develop for PC, Console, or Mobile platform as that will narrow your choices down drastically.
The main advantages of the PC platform are the wider choices for headsets as well as being able to add in third party input systems such as Razer Hydra (& Sixense STEM), Leap Motion, Thalmic Myo, Perception Neuron, PrioVR, Virtuix Omni, and tons of other input! Additionally both Oculus CV1 and HTC Vive have support for their own motion controllers called Oculus Touch and Lighthouse, respectively. It’s important to note that the CV1 will ship with an Xbox controller, and the Touch will be optional, whereas the current vive devkit comes with the lighthouse system as a default. The main downside of PC VR is the requirement of having a desktop class PC + being tethered to your HMD. Wireless HMDs are a long way away.
The mobile platform is good for targeting specific hardware (e.g. S6 class of GPU) and with much easier consumer setup. The main downside is the lack of positional tracking, input, and much slower GPU. These problems are being worked on, but it may take some time before a good solution exists.
The console platform is good for similar reasons to the mobile platform, but with higher processing power available along with better input out of the box.
Oculus DK2 - Ebay
Oculus DK1 - Ebay - Not recommended, it’s support is largely deprecated
Gear VR - Samsung - Need a compatible device (Galaxy Note 5 or Galaxy S6/S6 Edge/+)
Get an OSVR dev kit
Somehow apply for a Playstation VR/Morpheus Devkit - I’m assuming this one is through connections only.
Q1 2016 - Wait for Oculus CV1
April 2016 - Wait for consumer release of HTC vive or apply for a devkit, batch 2 (these are still very rare in the wild)
Wait for Apple VR
Wait for Unicorns