membership form for website

I need a form that asks for name and email address of the user.
Additionally, I want a form that asks users to input a username, but then checks that username to see if it is already taken or not.
If the user finds a username that is not taken, then I want them to be able to register with that ‘available’ username, but they are first redirected to paypal, because they have to pay first, and after I receive payment;
then the user is then registered with that username, or being the owner of that username. But they cannot register if they do not pay.

Also, it is not actually a membership site at all, and I just want them to pay for that username is all.

Sort of like when people register domain names at the registrars, and they purchase a domain name. That is what I am looking for.

I want them to purchase the username, is what I am saying.
But they don’t actually become members, but they simply buy the username. So they don’t need to login or have passwords or anything like that for now.

Also, I want the option of being able to send them an email telling them that they have successfully purchased a ‘username’ from my site.
But I have to be able to shut this automatic emailing off, in order to do this manually if/when I chose.

Sort of a complex form. Does anyone know of anything like this in wordpress plugins?