Unfortunately I don't know of any good way to do this within Apple Mail. There are a few close-ish things, but none are really solutions.
Automator contains the ability to create services—"contextual workflows" that can be run on specific items to process them. However, since it has no particular input format for emails, you'll likely be relegated to selecting the address or something of the sort, separately running the service, then using the output in your email. At that point, you're probably better off not using it.
Mail Stationary is, well, stationary. It's a template. However, their use is heavily restricted. First, they aren't context aware: unlike Pages templates, they have no ability to pull outside information and insert it. Instead, you've got to do all the work. Second, they don't work for replies, only new messages.
Outlook supports replies with specific templates via their more powerful Rules (which can be extended with third party extensions), unlike Apple Mail's rules (which can only be configured as basically an autoreply.) Gmail supports canned responses, albeit in a more limited format, as it exists outside the rules system.