This feature already exists, when you decide to buy a server by clicking "Launch VPS" or "Launch Server" you are brought to the configuration page, please see the link mentioned under the quote above.
But your configuration page isn't "fully customizable
[we're limited to only add extra storage]"...
- As it stands, we can't remove and add a piece of hardware based on our preferences
[you've got a lot of plans but you can fit more in-between each of those].