To use Fall Detection, you need an Apple Watch Series 4 or Series 5. The Series 4 is no longer sold by Apple, so I'd probably recommend the Series 5.
When Fall Detection activates it will call emergency services and alert emergency contacts (people you choose). To place the call, your Watch will have have some method of connecting to a cellular network. I'll assume you don't go anywhere with your iPhone- if that's the case all you need is the GPS model.
If you regularly forget your iPhone or for some reason don't carry it with you regularly, I'd consider a model with GPS+Cellular. Note that the GPS+Cellular model is more expensive and typically requires an addition fee from your carrier.
So armed with all that knowledge, I'll link you to where you can buy Apple Watch Series 5: https://www.apple.com/shop/buy-watch/apple-watch/aluminum-case
And some more information on the Fall Detection feature: https://support.apple.com/en-us/HT208944
Hope I was of some help!