I would start buying some brushes and supplies you can use at your barn with your own money that you can use on any horse like lead ropes,brushes, ect:)
I know not the answer you are looking for - but buy a cheaper nice pony and train it for 6 months, showcase the pony and sell for more money. This will demonstrate your commitment as well as help you put more money down on the $$ horse.
So them his qualities what are point out the all the bad things and compare them to one or two good qualities he has. Then talk about what you like in him and then ask your parents what they like about him.... Hope this helps and hope you get that pony. Good Luck to you.
The worse thing u can do is to beg for a horse so what I would do is just start taking about him a lot like " don't u think its a good idear , coz it's a really good opportunity " 😂X
I have good reasons like u won't have to pay barn fees or ANY other fees for rising someone else's horse