Now I'm curious... What WOULD be the characteristics for a body donor? Would it be similar to organ donors? Because I mean - they obviously can't take the body of someone who died of old age because that body is no longer reliable. Neither somebody ill, because the illness could also kill the head. Neither somebody completely healthy because that would be unethical.

If it is anything like a hand or face transplant you need the following:

1. The tissues must match and be compatible for the transplant. This is the same as any organ donation.

2. The physical size of the donor must be within a tolerance determined by the transplant team. Not too tall, not too short etc. Big people have big blood vessels and the mismatch at the connection point can and does cause issues. Tendons that are not similar in size won't work correctly. With a head transplant, I would assume that the spinal column has to be the same size in both head/neck and body. There are also size related issues to the windpipe and esophagus that need to be considered.

3. The physical condition of the donor parts must be compatible. Skin color, hair, age even tattoos all have to be given consideration. A pale white man's head on a dark skinned female body would not work for multiple reasons for example. You can put a female donor's kidney into a male body, but for external parts you want male to male, Caucasian to Caucasian etc.

4. The mental state of the, what are we calling the head guy... donor? Body recipient? The head dude must be in a mental state to stick with their drug regimen to reduce risk of rejection. A rejection of the donor would be fatal in this case. Then there is the mental strength needed to go through the physical therapy to learn the new body. Hands will react differently, arms will be slightly different lengths than the previous host, legs and knees different and so on.

That is off the top of my head, and I bet I'm not even thinking of the deep mechanical issues that the transplant surgery needs to deal with like the spinal fusion, or that as far as I am aware nobody has been able to repair a fully severed spinal column.

Then again - I don't know how long a brain can actually live for...

At least 122 years

There are still two people left alive who were born in the 1800's. And that is your off-topic brain bending fact of the night.