Hello and Welcome to the Horse Forum!!!
I'm no dressage expert, so I really can't help you there, but I did want to put in my 2 bits on the immunization:
In my opinion, its dangerous to not immunize a horse. Not only is it a danger to that individual, but it is a danger to all other horses around him if he were to contract a disease. Especially if you will be boarding this horse, you more than likely will need to get him vaccinated. If I were a barn owner, I wouldn't allow a horse to stay on my property without the basic immunizations. Its not only a danger to my clients horses, but a risk that could damage my reputation as a BO if an outbreak were to occour.
I'm not saying not being immunized is a deal breaker, but if you were to go with your first choice, I would look into getting him immunized and inquire about what happens when he is immunized.
As for the second horse, no horse is to old to train. If she didn't do too many trails with the horse, he may just need more time out on the trails. Usually the more a horse is exposed to, the less nervous they get.
Hope I helped you some, and good luck with your first horse!