Right approach in doing this signup process

I have a client who wants me to have 3 steps when a user signs up on our wordpress site.

On the first step.. There will be a domain availability checker. If the domain is available, they will proceed on the second step.

On the second step.. There will be fields such as:

• "Email Address"
• "First Name"
• "Last Name"
• A Theme/Template selection

Third Step..

PayPal Integration. This will be the payment page.

As of now, I am developing this one with the use of these plugins:

• Pro Sites
• New Blog Templates

The only problem I have or shall I say a question in my mind is that.. what is the right approach in doing this signup process?

Should I edit the wp-signup.php? Can the wp-signup.php do those steps I mentioned above? Is that too hard to do?

Should I download a plugin that would easily modify the wp-signup look? But would those plugin work correctly with these plugins: "Pro Sites" and "New Blog Templates"?

I'm not that good in Wordpress or shall I call myself a newbie, but I think Wordpress has a lot of hacks or I just don't know what i'm talking about.

If you could provide a code or what that will be greatly appreciated!

Thank You! :)