Since you're both into rescuing animals, why not visit a few animal rescue facilities and see what they have available or in training. If you find 1 that you think you might want to rescue and it has to go through their training before you can bring it home, why don't the both of you try volunteering at said facility so you can spend some time with it and be sure it's the one for you.
If he's so big on rescuing animals I'm sure he can sacrifice a few hours a week, maybe he'll get to see you work with the horse a few times. A free horse is not always a safe horse, nor is 1 in need of being rescued. Both of you volunteer and interact with the horse before you decide on rescuing. If he has your safety in mind, if said horse is not safe, he will mostly suggest helping find you 1 that he don't think will hurt you, even if it has to be purchased.
My appy was rescued by my riding instructor. She's also a trainer (level 5 apprentice under Dennis Reis). I paid no more for him than what rescue agencies charge for a re-homing fee. I spent a lot of time with him before and after I became the owner (sometimes he thinks he owns me). I went there during the week to groom, ground school, or maybe just put a rope halter and lead rope on him and walk him around the trails. I even took a lot of my riding lessons on him after she got him advanced in his training to where she thought I could handle him.
As for an older horse that has been there done that, around here a lot of them can be purchase for no more than what you would pay for an adoption fee at a rescue. At most rescue agencies, you still have to pay an adoption fee. So look at it like this, when you pay the adoption fee, you still BOUGHT the horse.