Hello, we are a group of experienced web developers who are ready to start on your site! We can easily create a new theme for a CMS, like Wordpress, which will make it easy for you to update, maintaing and edit. We will also write plugins to allow you to have users register and be able to see past invoices, for convenience! We can meet all of your specifications, provide support, host the website (which it doesn't seem you need, but we can still do so) and get it done in a week or less.
Please don't hesitate to PM us with any questions you may have! Thank you for your consideration!
-TwirlieGroup