Only the GPU. The specs are fairly basic and I want to build a new rig, rather than carry anything over. I will probably give it to a friend to add a cheaper GPU and game with me
Well unless you are going for a K series chip there is no reason to go for a Z series motherboard. In fact if you aren't going to overclock it would be better to go with Ryzen. Also NVMe will have almost no impact unless you are constantly reading and writing to extremely large files. For most people standard SATA is better and it would be better to go with a SATA SSD for the higher capacity for the price. But even then you can get more storage for cheaper and it still be NVMe in case you do move large files often.
In the end you will save about $100(I type with the dollar sign but I mean GBP) and get similar if not better performance.
You won't be able to overclock on the 8400, which means you can go cheaper on the cooler and motherboard. Assuming you want to keep the RGB liquid cooler and be able to overclock, a switch to AMD may be worth looking into. I gave you more NVME SSD storage too, instead of needing two seperate SSDs.
CPU
CPU Cooler
Motherboard
Memory
Storage
Video Card
Power Supply
Case