I'm wondering if you've made a decision about Thunder?
From his photos, he seems lovely: He could stand to lose weight. Maybe that would solve the saddle fit issue. He doesn't look so big he needs draft sized anything. I have a 16'2 Perch who uses X-wise English saddle, no problem. A thin pad, and off we go. Her bridles are all X-large, and a size 6 bit. Do not worry about shoeing, their feet don't require it, and it's so expensive. A good natural trimmer, not the extreme Strasser method type, will keep them strong and healhty. Standard Perch head, nice quarters, solid feet (they do need a trim), good attention to his handler. That is an amazing tail for the breed. Docked or not, they don't grow much tail. She seems intimidated by him, as others have pointed out. I've got a good bit of experience with both Percherons and TB crosses, and find that though they are huge in body, they are not huge in their minds. This is not to say that they are dumb, or eve slow, but that they are calm, biddable, and respectful, as they are bred to be. SOmetimes it takes a moment or two for them to respond to a request, unlike a TB, but they will respond appropriately.
Things you need to ask:
sire/dam
reasons for the temperament rating of 5
exact duration of training and exercises covered in training
name and contact info of trainer
once you contact the trainer, find out:
the worst thing he horse has ever done (this is so telling)
the best thing the horse has ever done
will he stand for the farrier?
will he stand for the vet?
does he cross tie?
does he trailer load?
About the spooking: this is usually all about confidence. Once a horse has the confidence he needs to walk out, ride out with his partner, spooking goes way down. Confidence in horses, I believe comes from their being able to think, rather than react. Training a horse to take his time, think about what is being experience, what is being asked, is the most valuable tool against reactivity, i.e. spooking.
Also, draft horses seem to exhibit a lack of awareness of both their hindquarters and their feet. They simply do not know where their feet are. When you don't know where you are in space, and something triggers your fight or flight reflex, you are more likely to panic and react, rather than stop and think.
With that in mind, I would suggest taking a look at some of Linda Tellington-Jones' exercises in the Playground for Higher Learning, and some of the TTouch stuff she does. TTouch for trust and awareness is especially helpful for draft animals. This is from my personal experience.
Just my two cents. Please let us know what you decided!