I would pass.
As one member said, the sheer size of an 18.2hh horse means more health issues, particularity in the legs.
I did this once. The pony sounded perfect. The owner sent us over 20 videos of her competing, being hacked out at home, being tacked up, schooling. She looked and sounded perfect.
The thing we didn't consider was that she could have been sedated in every video. We thought she was just laid back.
When we got her home, the sedation was starting to wear off. We got her out of the horse box, and then we knew something was desperately wrong. She was literally stone mad. No amount of work could help her. Every single person who got up on her was thrown off and nearly killed. I never even rode her.
In the end she was put down. She was just too dangerous.
If this happened with an 18.2hh horse, you wouldn't be able to control it. A horse of that huge size could be extremely dangerous.