Advice on what to look for when loaning a pony with view to buy?

Look for someone you can trust who is experienced with horses (a trainer, a mentor from a local horse & pony club, etc.), who can guide you through this process and also see the individual horse and its behavior. It doesn't sound as though you have much experience with horses and ponies (Please don't take that the wrong way, as everyone has to start somewhere and I'm excited to see anyone get into such a rewarding interest!), and I would definitely doubt any advice given to me by someone who would lease or loan a pony to a family with such little previous experience, or someone who would loan a horse out with tack if that tack was an ill fit for the horse.