Thoroughly examine the front and back of each item of leather looking for any significant cracking. My marathon harness was used hard and put up wet before it came to me and the padding on the breast collar is cracked but its still structurally sound.
Check all of the buckles to be sure they are stitched in securely and that they function (tongues aren’t rusted shut, etc). Also look for buckle tongues that might be excessively bent.
Check the adjustment holes to make sure they aren’t cracked or the holes too stretched out. You do not want to use a hole that is damaged.
Fit is important, and its basically common sense. The saddle may or may not have a tree, people have mixed feelings on trees, I’m pro tree but I also have a special needs horse. What is important in any saddle is that his has some gap for the spine. You really don’t want to have the saddle sit directly on the spine.
Check the bridle too, thats a big one that can be tricky to fit.
Little things like tug style and trace style are not critical and can be changed and adjusted when you find a vehicle.
This site is pretty good for the names of harness parts:
http://www.discover-horse-carriage-driving.com/harness-parts.html
and how to fit a harness:
http://www.discover-horse-carriage-driving.com/fitting-the-harness.html
http://www.discover-horse-carriage-driving.com/driving-video.html
How exciting for you both, good luck!