Dark Souls 1 is 100% worth playing. I won’t be a purist and say that you need to play it before 3, because they’re mostly standalone experiences… but you might miss a handful of references if you play 3 first.
As for how to play: if money’s not a question, buy the remaster – you’ll get a better online experience that way.
If you’re on a budget, however, you could simply Install the DSFix mod and start playing with the adaptive controller as-is. There’s unfortunately no in-game rebinding UI, but you can still do remapping using either the Xbox Accessories app or directly in the Steam overlay (requires Steam to be in Big Picture mode).