Dear Polly,
It's lovely to hear that you have decided to give cloth nappies a go! Apart from the benefit to the environment they'll also save you a lot of money and are really easy and convenient to use with the modern styles available.
I'm not familiar with the GreenKids range however there are many brands and styles available so the choice is really which appeals to you, which you prefer the style of and of course pricing. The material that the nappies are made of will influence the weight of the nappy, absorbency and also the softness or feel for your bub. Bamboo for example is highly absorbent while still being soft and gentle.
Inserts and boosters do increase absorbency and can be especially useful for nights to help extend night sleeps so your bub doesn't wake up early because they have a soaking nappy. Let's face it, we don't get enough sleep as it is, so anything that helps our little angels to sleep comfortably is worth considering. However in terms of leak proofing this really comes down to the fit. If it's a snug fit then cloth nappies wont leak which is just the same as disposables.
Hope that helps and good luck with your new cloth nappies!

Kind Regards,
Fiona.