RE: No products showing when adding 'Existing Product' to a quote line item
Hi,
Please check if the products are in DRAFT state. If Yes, Please Publish the Products from the individual form or from Advanced Find as shown below:

You can also opt to create products in system in Active state by following below steps:
Go to Settings --> Administration --> System Settings --> Sales Tab --> Select "Yes" in Create Products in Active State.

Thanks,
Ajit