Buy the one you can afford, keeping in mind to get one with legs so you don't have to visit the well again soon, and have something you can grow into if needed.
Better yet, one of each. After all, the broader support range, the larger the u$er ba$e. Takes coin to make coin.
I would start with a series 5, to be sure to be able to use new OS versions, which will likely add capabilities in health domainj. May be in sleep monitoring as well…