I highly prefer Godaddy.com for my domain names. I've been using them for over 3 years now and have never had an issue with them. Sure the price increase got me a little miffed, but other then that, I trust them with my domain name. =)
About the package/billing/domain registeration script, I would go with our free copy of WHMCS!
Free Resources and Tools at Surpass Hosting - WHM CS
With WHMCS you are able to import all of your accounts from WHM to WHMCS and also packages already made. Like you asked above, you are able to set the settings so that accounts can be created automatically after sign-up or payment approval, of course there are many other options for this.
There is also a feature for domain registration with a list of domain reseller host for you to use.
So I highly recommend you to check it out and download it! We also have quite a few tutorials and articles in regards to WHMCS within out web hosting tutorials and search me boards here on the forums! Good luck! =)