I’d keep the ride, we find the best ones when we aren’t looking for them and he’s a known quantity reducing the gamble of buying an unknown. You look fine.
BUT, BUT, BUT…when it’s not your horse you really are not in charge of important and necessary decisions, the relationship with its owner can sour or they can sell it out from under you if they need to and it’s their absolute right to do so with it without your consent.
You need to get some sort of a lease agreement in writing until you can buy him.
I know owner is your BFF and would never pull him out from under you as a reward for all your work making him better and more attractive to buyers even if she lost her job, got sick or needed to sell ASAP. BS, get something in writing before devoting time, energy and your own money developing him and buy him to avoid almost inevitable heartbreak and conflict over decisions concerning the horse that are the owners to make, not yours.
BTDT several times, watched many others make the same journey to the same unhappy ending, don’t go there. Never ends well.