Wordpress Site Migration Strategy
The Wordpress community has some thoughts about the Wordpress Site Development and Migration Strategy.
Wordpress Domain Mapping
A client may need to have multiple Wordpress sites installed in a single organization. Here's how to add multisite support for Wordpress and how to perform a Wordpress Multisite Installation.
You might need to familiarize yourself with the Wordpress subdomain vs. subfolder strategy and the Difference between Subfolders and Subdomains.
If your client wants multiple domains, they might need Unique Domain Name Mapping for Wordpress Multisite.
As with many needs in Wordpress, a plugin is available for Domain Mapping, and an alternative approach can be explored for Domain Mapping via the Wordpress Codex.