Depending on your daughter's experience, I would not be buying a 5 year old horse to start eventing with ... but rather an early teens horse that has successfully competed, and is safe for beginner eventers. Eventing is comparatively VERY dangerous to other disciplines, just because often, you are riding much faster, and going over solid fences. No matter the height, if the younger, inexperienced horse looses his confidence, refuses a fence and throws your daughter into it, or even catches a leg and falls onto your daughter ... You will be wishing that she gained basic eventing experience and stickability on a horse that knows his job.
If her trainer believes that this horse is confident enough to be trained in eventing and learn with your daughter, then that is a whole other matter - but if she only has experience with older lesson horses, I would really think hard about getting her something a bit older, even just 8-10 years old, to make sure they have all of their silliness out, and have the right amount of confidence for eventing.
Another thing to keep in mind with Warmblood types, is that some of them can't handle longer cross country courses - especially if they are purely bred for dressage. The lower levels should be fine, but you typically see TB's or crosses in the upper levels, for the speed and endurance that comes with their breeds.
Happy eventing! I miss it, so much.
Toofine - 1998 Half Arabian
Minnie - 2013 Morgan