I am a 66yr old nurse, still working. Have an elderly saddlebred-pinto mare who won halter ch in earleir days. Still gorgieous. Also a quarterhorse, supreme working ranch horse. Have had both knees replaced & am just starting to ride again. Need some moral support.
I just read your "wall". I also have a client who's a newly retired vet, who's had both hips replaced. She's 61, and training her second dressage horse. Either or both of your horses will work for you to start with. They might even do you through your whole dressage career - one of the great things about this sport is that breed and colour don't matter. It is important that you be comfortable, and feel safe, so if neither of your horses fit that bill then try to find a coach with horses you can ride and/or lease, or find a lease. That can be a a really good, less expensive way to get a good horse. A solid Pony Club horse would be super - something that's been grown out of, but that the family's not yet ready to let go.
Where in the US are you located? Some of us might be able to assist you with finding a coach in your area.