Watches

I am trying to find an appropriate watch for my son. It needs to have cellular capability and be able to track him. He is autistic and a runner. Winter is coming, I need to be able to find him quickly when it is -45°C outside. And he needs to be able to contact me when he’s realized he is lost or can’t make his way home.

I’m a little confused as to why they seem to be a cheaper price on this site then they are to buy one used. I don’t know what I missing in the information. Is the total cost of buying a cellular plus GPS watch with the band $429+tax?

And Why can’t I find any of the series 4 or series 5 on the site to find information on them. Rogers, my cellular provider, told me I can get anything above the series 3, which is not supported in my area. Would this include the SE series?

The series 4 and 5 seem to be widely available as used for $500 to $800. But I can’t find info on them? These are older models them the 6, which I can’t afford, but what is the SE is it better that the 4 and 5 series? Lol I’m just confused as to why they seem to be more expensive used than the SE available through the apple sight. What info am I missing?