IMO soundness is all-important but a slippery thing. An adult horse can hurt himself permanently the very next day after you've purchased him. Who knows whether the foal will develop OCD or not? And so forth.
I believe it's all about time and money available to you.
Question is: When do you want to ride? Now? Get an adult horse. Ta-da!
If you can wait years to ride, then the question becomes whether you have the finances and appropriate resources to do all the necessary support until the horse is going under saddle.