Hi Bill,
Please review the following link for additional license information (Phone and Voicemail) for UC500:
https://www.myciscocommunity.com/docs/DOC-1445#nbsp_How_many_mailboxes_are_supported_on_the_8user_16user_24user_32user_and_48user_models_of_UC500_
You can use an 8U system, but your statement is not completely correct though. You can have up to 10 VM licenses on an 8U system, so you are good using that. Go with an 8U license for now, and purchase a 16U one in the future when your customer's business grows.
Thanks,
Marcos