Ok so my 2 cents…large horse=large bills, broad scale wise…feed.hay…etc, large horse=higher fall point for a rider,= bigger bills. Large horse=bigger bones to break, bigger muscles to try to work thru a buck or her fighting you on something.
I know you said money is not an issue and ya I am a lil jealous lol. But, do you really want to risk spending that money? If she is broke, how is this mare going to look come spring with the feed and hay costs she will incur? If you say sorry no deal in the spring is she going to try to subtly guilt trip you about it until you cave? I get that she cares about the horses and wants them to go where she still sees them and knows they are cared for. However in all honesty it sounds like she has found a way to hoard her horses and make money off of them lol. Idk how tall you are, your age, or how long you have been riding but if this huge mare gets hot, can you reasonably control her? She is calm now, what about the spring crazies? What about when she goes into heat and the ‘mare’ really comes out? If the arena is outdoor how much will her hot blooded young mare nature cause spookiness and risk your safety more than need be?
Not that she is a bad horse at all, those are just the first few worries that pop into my head. Can you just do a shareboard type thing thru the winter? X amount a month and a lesson a week on this mare or something?
Please keep in mind, even if you have unlimited funds, you only have what you have now for body parts, bones can only break a small amount of ways, nerves can only be damaged so much, muscles bruised or tore only to a certain point…before all the money in the world still cannot fix those body parts enough to sit a horse safely or happily again.
Whatever decision you make still wishing you the best of luck, jingles!!