Thanks everyone for all the great responses!
I just found out the barn I had my heart set on stabling at is full... with a waiting list. Boo! So, that's hurdle number one: find out where I could keep her. My other two horses are at my parents' house (along with my SIL's gelding) but they live over an hour away, and we have all geldings so I hate to disrupt that balance. (We brought a mare in once and it was somewhat of a disaster). Additionally, I would need a trainer and there really isn't one nearby. So... I guess I'll look around for another barn and put my name on the waiting list of this one.
I definitely know the value of a good saddle... but I managed to make do on mediocre Western saddles until I finally "traded up" and found a nice, used Circle Y on Ebay
for $300. I'm all about the bargain hunting... but I find it a little silly that a rescue is sort of being judged for not having a "proper" dressage saddle. Chances are, that saddle either didn't belong to them (possibly belonged to the girl riding the mare) or was donated to them. This is a (very good) rescue in mid-Wisconsin and they deal with a lot of Western horses.... haven't seen to many dressage horses/dressage prospects listed on their site so it would be a bit silly of them to go out and buy a dressage saddle just in case another horse like this might get donated.
As far as a trial period, the rescue gladly takes back horses that are adopted out and just don't work out for whatever reason. (In fact, I think it's a requirement: if the horse doesn't work out, you have to return it to them). I would feel a bit bad about asking for an adoption fee back so I'd probably just let them keep it (adoption fee is only $500)... but honestly I would likely just see if she can go another direction if dressage isn't for me. :)