Thanks for the advice guys! Ok then, so I'm going to go find these various batteries and the Nitecore i2 charger.
Now then, I have some options. The cheapest I've found the Sigelei ZMax V5 on it's own is £59.99 at vape escape. It doesn't mention if it comes with the extended bottom cap. I assume it does.
So that's only a fiver cheaper than the full kit. For £5 more (£64.99 at innovapours), the full VMax Z5 kit also offers a protank style clearomiser and a free 6ml bottle of juice (and extended bottom cap). Would it make sense to buy the full kit then, but discard the charger and batteries and use the ones you've recommended?
Or I could just buy the ZMax V3 standalone for £49.99 also at vape escape, which says it does come with the extended bottom cap.
I'm not sure how useful the mobile phone charger would be to me that comes with the V5, so other than that, is the V3 just as good as the V5?
edit: The Nitecore i2 and i4 don't mention compatibility with 18350 batteries, only 18650.

It says Compatible with: Li-ion 26650, 22650, 18650, 17670, 18490, 17500, 17335, 16340(RCR123), 14500