I want to start building a photo library on my Lightroom CC account, once established and looking good, I want to encourage the client to subscribe to Lightroom CC then transfer the whole photo library so they can continue to manage, add and edit images - is this possible?
Your library has nothing to do with your CC account. The library 'sits' in your local catalog.
The only thing synced to the cloud or mobile are the collections marked to sync.
That said, you can copy/forward a catalog (together with the original photos!) without any issues even to non-subscribers that use LR6.