Need help to find the best choice for the research experiment between Focus 3, Pro 2, and Pro Eye

[Marzieh]

 

1.  Is it right that Focus 3 is a stand-alone device and depends on a battery that lasts for a maximum of 2 hours? are there any other options for recharging for running experiments? 
2. Since Focus 3 and Pro 2 do not have eye tracking functionality like  HTC pro eye, Is there a possibility to add eye tracking to them?
3. Which one is more comfortable for research experiments on people? We have participants of different ages, so I want the most convenient one in terms of reality, immersion, weight, etc.