Dexter sound like a great dog and a lot of fun to be around.
I don't want to come off wrong when I say this because I am very much supportive of rescue groups and I bow down to those that take on pups with issues, but it also speaks to why some people want a puppy and not a rescue. However on the other hand some dogs regardless of good breeding have issues, my last purebred Kirby, came from a good breeder who was known in the show ring for having nice dogs, however he was a 4 lb ball of angry aggression, that was not happy unless you were playing tug of war with him, but you had to be careful playing with him because he would bite hard to get his toy. He also showed absolutely no affection toward me and had lots of weird hangups. The pair of yorkies I had before him were like living teddy bears and I spent years with them visiting nursing homes, they would look at me like I hung the moon, Kirby would not even look at me
Now I have Lola a yorkie mix, when I got her I was not sure what to expect, all I knew is that Kirby came close to completely turning me off yorkies, so I wanted something a little different. Lola has probably the best personality of any dog I have ever had and I got her at 4 months old from what I realize now would be considered a broker. Sometimes it's the luck of the draw on the personality of your dog, but I think it helps to socialize and train them from an early age.
You will enjoy doing the therapy, I am assuming you will visit nursing homes and so forth, the people really enjoy seeing the dogs, especially if they can do tricks or wear cute outfits. YOu know I think I will look into it with Lola, thanks for inspiring me.