The SKX has historic value but is a crappy proposition compared to a modern diver – old albeit reliable movement and no hacking – at that cost. A citizen promaster, Seiko Turtle, Sumo or Samurai are “better” at that price point. Try to find a version on Sale that has Sapphire Crystal, like those with a “King” prefix e.g. my King Samurai is ISO rated, Sapphire and ceramic bezel and was £380 on sale.
Not withstanding Seiko are about to launch the new SKX in a month or so.
The 5 will do everything you need, but isn’t an ISO diver and is a “cheaper” proposition – and entry level ish watch if you like. That said, you’ll get many design variants to chose from and strap swaps are easy with drilled lugs. It’s all worth bearing in mind that Seiko have a tendancy to understate specifications, many have been taken deeper with now issues but that’s at your risk of course.