Clyde: With all due respect to someone trying to get started in landscape lighting, your initial post causes skepticism among most of us as to your readiness for this project (as simple as it may seem to you.)
First, you say the client has chosen Top Dog fixtures. Most of us wouldn't be dealing with a client who chose what we were to install. Second, you are not aware of products enough to tell the client that this fixture is not appropriate for the intended use. Lite 4 pointed out to you that this fixture is not made to be turned upside down for downlighting.
Then you ask us to tell you what the typical wattages are for transformers.
I would suggest you look online at the major manufacturers to see what is available. Despite your electrical background, you don't seem to know how to add up the total wattage or VAs of the lamps you are using plus wire resistance to find what size transformer you would need.
Later, you say you may attend a class put on by a manufacturer. Those types of classes are OK to pique your interest but don't expect to learn anything but the most basic information. You will need to do much more studying.
And then you can start with some simple jobs.
Please accept my comments as an attempt to be constructive.
Starry Night Lighting
"Always look on the bright side of life."