What age and gender of horse is the best, in your opinion, for a first horse?

Generally a gelding over the age of 12 years old. Your size could do 15hh and up easily. Maybe 14.3-14.2 but anything smaller than that would not work well. For what you want either a QH or Appaloosa would be good. Arabians can be good but they are generally hot blooded and hot headed and not good for beginners.