Nothing

This is a complete system and liquid is already inside the system, it has fan and termal paste applied. You just need to make sure you have enough space in your case for this. I use it with Streacom DA2 (Mini-ITX). If you could not connect on the first try and termal paste gets smudged or if you want to remove and install again, you can buy additional termal paste just in case.

The kit includes pretty much everything you need. There is thermal paste pre-applied to the cooler, so you don't need to buy that. You could get a quieter fan if you want, but not necessary.