It's probably best to get a wide variety of standard gauges just to see what works for you. Sometimes gauges you wouldn't think to use end up being the ones seeing the most use. Go for the standard set of 32 - 28 - 26 - 24 - 22. 50 feet of each is plenty to play around with. Those are the main gauges that people use. When you see someone talking about a build that you might want to experiment with, they'll likely be using at least one of those.
For claptons, super-thin gauges are the best outer wire. I'd recommend starting with 34 and working your way down, though. As noted by
@martinelias, they are harder to work with. Past 36g, the wire starts getting as thin or thinner than a human hair. I think a good clapton set would be 34 - 36 - 38 - 40. With these, don't skimp on footage. Buy at least 250 feet of each. It can take 15-20 feet to do enough clapton for two coils.
When you see "n" next to a wire gauge, that usually means the wire is nichrome. You'll see a lot of nichrome users in the clapton game. It's worth looking into. It heats up a bit quicker than kanthal and ohms lower. Claptons can usually benefit from nichrome as a core because of the extra power you get per unit of mass. It's not absolutely necessary... ...just a way you can go with it.
As to your last question, I do claptons on all 3 of those guages. Which one I use depends on the power and ultimately the airflow I'm constraining the build to.
