Thanks for your questions before signing up. The CPanel VPS does give you full access to WHM, including the ability to create resellers under your root account. These resellers would also have WHM access to manage the sub-accounts that they create.
Both Disk and RAM are upgradeable in the same product line, but you cannot add more than double the base amount of either. This means that you could upgrade to 20 GB of disk, but not more. However, we can do a no-downtime migration from a 200 to a 300 or 400 as long as you stay with a CPanel VPS.
Most customers have found that it is more economical to upgrade their base package to a 300 rather than adding both diskspace and RAM to the 200.