I got same questions.

The sky is the limit with elgg.  If the question is "Is this possible?" the answer is always going to be yes, but you will have to implement it yourself.

There are probably a dozen different ways to attack this problem, so I'll just share what I did.

First, I have a custom registration form that includes some more information (including billing info).  Then in the registration instead of sending them an email validation query, I make sure they are set as NOT validated.  I use paypal for payments, so I have my paypal transaction fire, once I get the result assuming it's successful, I mark them as validated, and send them a notification as well.

Start by looking at the existing registration process.  actions/register.php handles the form once it's submitted.  The form itself can be found in /views/default/account/forms/register.php.  Use those as the basis of your new plugin.

There's a lot more to consider.  Make sure you're using https for taking any credit card or privileged info.  Don't store CC info.  Give them an easy way to cancel or get help/ask questions, etc.