I never intended to horse shop, and most definitely didn't have a "zing" when I saw my now-horse Sky.
He too was a failed Therapy Horse. Spookiness is a complete understatement.. he once tore down a fence, an old barn, and broke every halter he owned. And that's just before I met him. While I knew him, he bolted, spooked violently, rubbed someone off on the fence, ran away with someone and they consequently broke their back (but recovered, we're still in touch), tried attacking the farrier, and killed a few pairs of crossties.
Needless to say, he was a MESS. I despised him because everyone thought he was pretty and spoiled him with treats. He was muggy because of it, which just further set me off.
I thought I was a know it all (ha!) and for some reason my then-boss and trainer put me in charge of lunging Sky, her personal horse due to the failed therapy use. Then the same day he broke someone's back, she had me get on him and ride him out of his spookiness. Besides being a giraffe, he was fine. I barely knew how to post at the time
Fast forward, I began to lease him and I had no real thoughts of buying him but I did see him as a challenge and he became really interesting to work with. We began to bond, without me seeing it. Many people tried to buy him but he was always returned because he was uncontrollable and unpredictable. I kept leasing him until one day my trainer gave him to me because she saw what I didn't see, that he had chosen me.
He was definitely NOT the horse to buy, he barely stood for a bath let alone let you pick his hooves. Putting on polo wraps was a marathon of trouble... let alone his manners under saddle (remember I didn't know what I was doing, and somehow managed not to die)
So I can't say for sure what indicators lead to ending up with the perfect horse. He's exactly what I need, but in the beginning he was FAR from it. A horse that sounds good in an ad may not be the horse for you. That dead broke horse may not be the horse for you.
Horse purchasing is a gamble, because it's not a car that stays stoic throughout, it's a being that has the ability to be influenced and change. Just as with people, not all click.
My advice? Lease before buying, just like the OP is doing.
"Strength is the ability to use a muscle without tension"