How to calculate the required characteristics of the virtual server?
Accurately calculate will not work.
We all have different sequencing, different priorities are configured for virtual machines, different storage configurations. Different oversell. The different ratio of memory, CPU and disks. In the end VMS with identical characteristics from different hosts may differ in performance.
The most reasonable in this case roughly estimate how much memory IAPSO, disk space and CPU and guided them around to select VMS from a specific host.
If resources will be missed - not a problem to switch to more expensive tariff plan if it will clear the excess - go for a cheaper rate.
You just need the right to negotiate with the hosting company, to discuss how hassle-free it will be to add resources or migrate to another tariff, and how it will look at the price.
Most hosting companies provide trial periods - even if on the website there is no such, you can negotiate.