My disabled wife knows that there are a thousand plus places she will never get to,

Thinking about buying a VR headset to pair with Sony Xperia XZ Premium so she can get to visit some from home.

What headsets should I be looking for?

What a good husband you are! What headset you get depends on your computer, and/or your phone (or the wife's phone). HOWEVER, perhaps the quickest and easiest way to get her connected to VR, would be the Oculus Go headset which does not require any computer or phone. It costs around 200 dollars. It must be initially started with an Android or iPhone, BUT there after it works without a phone or PC. It supports the use of wired headphones and I DO suggest you get a good pair, it really helps with the experience.

IF you have a high-end PC with a good graphics card such as the Ge Force 1060 (or above), then you can consider the Oculus Rift, HTC Vive, or a Windows Mixed Reality headset. These are more advanced, BUT also more expensive. I don't necessarily recommend going this way over an Oculus Go.

As for the phone. Her phone does not support, the Samsung Gear VR (requires high end Samsung phone), or the Google Daydream View. These are also great options, but won't work for her phone. What her phone DOES support would be a Google Cardboard headset, now these do NOT have to be made of cardboard, you can and should get a plastic model, such as the Merge VR. Again though I would recommend the Oculus Go. The price is reasonable and it works VERY well!

PS THIS is the app you want! ......

https://vr.google.com/earth/

That's Google Earth VR

I can also recommend the 'Discovery VR' app, which is more thrilling than the Google Earth application. It's an app of the Discovery Channel with great destinations, even outta space. :)