Those VPS work with the following idea
1- You start with initial disk space as set in your plan
2- Daily you will get some GBs added as disk quota in your vps (you can see it in your client area)
3- When you reboot your VPS the disk quota you accumulated will be applied to your vps and the disk size will be bigger by the same amount of the disk quota before reboot
Your quota will keep increasing daily indefinitely as long as you keep the VPS active (paid for)

No, the server will never run out of space because all our storage offers we decoupled storage from compute servers,
that mean usually your VPS is running on a server while your storage is actually in another 1 or more servers (servers specially built to have many disks with enough power and cooling),
The hypervisor (the compute server) run NFS to access the storage servers and offer the storage to the vps
That mean we can have a VPS with 500TB disk space on a compute server that have very little disk as long as this compute server can access 1 or more storage servers with enough space for 500TB

We add storage to the VPS in blocks of 2TB disk max, that mean you can have a disk in your VPS that start with size of 100GB for example. And when you apply your quota the size will keep increasing in that disk till it reach 2TB at that point any new quota will go to new disk .
Each disk can be in different storage server so when we are low on storage, we simply add more storage servers and so on

All our storage plans come with raidz2 storage (raidz2 is the equivalent of raid6).
We usually run our array as 6 or 8 disks 2 of them parity disks that means between 25% to 33% of our disks is used just for protection.

The price is divided into 2 parts
a. Initial costs like hardware, ips etc
b. Recurring cost like power consumption and bandwidth
The plan is priced in a way that it will cover the initial costs within our ROI timeframe plus extra
This extra is calculated that it can handle the initial costs of all the hardware that we will need to increase the storage within acceptable ROI as well
For recurring costs, the Bandwidth is constant on the plan so it will not increase plus bandwidth price do decrease with time and volume
For power consumption the price handles the extra power consumption used by extra disks which is very little and the power price do decrease with volume as well

yes, you can and the new user will keep all the quota and gained disk space using the space price as you did