Managing and Billing through linode

No linode doesn't have any out of the box solutions it is an unmanage service which means you have to do it all yourself, however you can install cpanel/whm yourself.

If you plan on taking credit card details you will need SSL, however I suggest you pass your payment processing to an already PCI compliant vendor since it saves you a ton of hassle.

Braintree looks ok to me, although you need to be hitting at least a few thousand per month in charges for it to be competitive with a merchant account or PayPal.