Your best bet would be to use PHPCoin its simple and good if your just starting out a webhosting company then i would subject Clientexec when you start to get more customers as Clientexec is easy to admin and is pretty fast
You cannot find a good free hosting billing software. If you don't want to pay for a one when don't look a billing software. Then you can use a whois script together a form mailer script and a calender software for billing tracking.
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Be carefull using phpCOIN, this is insecure.
Once even their own forums got hacked and deleted! expert hackers can work with phpCOIN like a remote shell. lots of phpCOIN users insulted the poor developer in his forum because their sites got hacked through phpCOIN and they moved to other softwares like whmcs!