That goes in place of the cartridge, indeed. They have a pretty good lifespan, I've read people using them upwards of 4-5 weeks (I'm on day 5 with mine and it's still working like new) before they have to replace the head (which is ridiculously easy). You'll want to clean it out every few refills, but it's super easy and takes ~5 minutes. You'll (probably) only get decent vapour from the 1.8 ad 2.4Ohm heads, the high resistance one won't produce decent watts, though you may like that.
You don't HAVE to get anything right now. Kits come with everything you need usually; but there's a lot of other attachments you can get. They all perform differently and some people prefer one over the other. The only way to know which you prefer is to get them and try them out. That's why you've gotten so many different suggestions here, everyone has their own favourite and will suggest it. The best thing to do is to try out a few things, just buy one or two, and experiment. Once you know what you like you can just order more of them.