Problems arise if overselling is too strong, and at some point there is not enough memory for everyone. Then the systems use the swap file (swap file), that is, they access the disk, which has a very significant effect on the speed of work.
Today, finding a decent hosting is easier than ten years ago. The level of services provided by large companies has leveled off. Therefore, the recommendations that were relevant before have largely lost their sharpness.
In the Digitalocean cloud, it is possible to take VPS snapshots and then create new servers from these images. In this case, there are problems associated with changing the ip-address. You need to find all the configuration files where the old ip is mentioned and replace it with the new one.
The question, VPS with how many vCPUs to rent for a site, is not so simple. On the one hand, it is intuitively clear that two cores are better than one: after all, two people can access the site at the same time, and it’s good if no one has to wait.