With a good trainer, a horse with uphill tendencies and three pure gaits can be trained to third level or higher. Certainly, you may not score as high as someone on a fancy moving horse. However, for a typical adult amateur giant, flamboyant gaits are not always the best thing. You need to be athletic enough to ride those gaits. I have seen way too many amateur riders buy horses that were beyond their riding capability. The horses ended up being ridden by the owner’s trainer, sold, or ruined.
When looking at gaits, walk and canter are important. A good trot can be made a great trot with proper training and muscle development. I’ve brought along a number of young horses and none of them were terribly expensive or spectacular. Training made them all nicer and some went through amazing transformations. I’ve seen the same thing in friend’s horses.
It all comes down to your skills and goals. You will get good advice and varying opinions on a board like this. If you have access to help from an experienced dressage trainer who has brought along a number of young horses, ask them to help you. Evaluating young horses is not always easy. They can dramatically change as they mature.