I have a Series 4. It's getting a little old compared to the newer devices, but works perfectly well and does very little that the newer ones can't do. It doesn't have a temperature sensor or the oxygen functionality and it's the last model that doesn't have the always on watch face, but I'm not thinking about changing it any time soon. It's fast enough for me and it charges overnight, so no issues there.
It still runs the latest OS, but how long that will be for, only Apple knows and it is now the oldest model to do so. This is probably the thing that would concern me most if I was thinking about buying one, but it's not like it will suddenly stop working.
How long the battery lasts on that particular one depends on how the previous owner used it.