Control Panel: is webmin sufficient for our requirement? I heard that cPanel consumes a lot of memoey and CPU.
--Webmin isn't a control panel as it is a GUI to configuration files. If you aren't experienced from the command line with Linux I recommend either Plesk, cPanel, or Directadmin. Out of the three Directadmin uses the least amount of memory.
All you need to do is email our sales department and we will add the resources to your account and then either send you the prorated invoice or charge your card on file. Once this is done our support team will add the resources to your VPS.