Are Arabians good at dressage and jumping?

I am buying a 14.3hh arabian, and I am 5ft7, 16 years old. Firstly, is this horse big enough for me? I will ride her 3 days in a week, and will school her in the arena twice a week, one of these days taking him over some small fences, and on the third day I will take him out for a trail ride, whether it be in the rainforest or along the beach if I have time. In your opinion, how far in dressage (eg. what movements) do you believe this horse could reach, and how high do you think she could jump? I won't be competing often, maybe one show or 2 every 6 months. Also, she does tend to cat leap (has jumped 4ft) and is not very good at engaging her hindquarters when I ask her to go on the bit. In fact, she hasn't got a clue how to tuck her head in, etc, the correct way. Advice on the above would be much appreciated, thank you in advance.