Seriously why breed? I have a wonderful Jack Russell terrier that has a perfect tempermant and everyone begs me to breed her and have puppies, but really I don't want to put her through the stress. As much as I would like to breed her I know there are enough dogs w/o homes and I would not want anything to happen to her during birthing.
I don't know you or your horse but I def would listen to the other posters. Do you have the time and $ to devote to the mom and baby? Is there really a reason to breed the two horses in question, do you believe they will produce a really exceptional horse? There are so many full horse rescues, etc. Just really think about it. Good luck