According to the documentation, Kubernetes reserves a significant amount of resources on the nodes in the cluster in order to run itself. Are the numbers in the documentation correct or is Google trying to sell me bigger nodes?
Aside: Taking kube-system pods and other reserved resources into account, am I right in saying it's better resource-wise to rent one machine equiped with 15GB of RAM instead of two with 7.5GB of RAM each?
Yes, kubernetes reserves a significant amount of resources on the nodes. So better consider that before renting the machine.
You can deploy custom machines in GCP. For the pricing you can use this calculator by Google