Hello,
Yes, will not be an easy task to build a full invoicing system in Guru, we investigate this and will see if is better to have own system or to add a third party system into Guru.
Till now, you can modify the word 'invoice' from language files which are located:
root/language/en-GB/en-GB.com_guru.ini
administrator/language/en-GB/en-GB.com_guru.ini
search for it and modify it as per your needs.
Thank you!