The first part of the question seems to be outside of this forum scope. Its a DNS and Virtual host setup. Check bind and your web server documentation.

For the second one we use a couple of classes to send email through Ofuz, which I think should make it possible.
Assuming you have a bit of PHP knowledge.
You can find documentation on the Classes and method to send emails to contacts at:
http://www.ofuz.com/opensource/wiki/tutorial:send_emails_to_contacts

Then what you would need is to create a setting item in the Settings:
http://www.ofuz.com/opensource/wiki/plugin:setting

Let me know if the documentation make sens

Thanks. For the first part I didn't mean manage DNS records, but when I go to settings there is no way for me to set my Ofuz account to a sub-domain of my website. Does that make sense. So that when I send and invoice to clients it would have a link like projects projects.mysite.com/inv/invoiceurl instead of ofuz.net/inv/invoiceurl.

The rest makes sense to me. Thanks!

got it now.
Not at this point but that sounds like a good idea.

But for the payment page how will the ssl certificate be managed ?

I guess each domain would have to purchase an SSL certificate, is that doable? It only applied if you use authorize.net, correct?

Yes, because with paypal you get redirect to the paypal site