I'm 5'6" and I ride in a 17.5" Close Contact saddle. Depending on the style. In the Pessoa XP3 I need the 'Extra' flap, and in a Prestige saddle I ride 17" With a +4 cut flap, because I'm quite long from hip to knee. So it depends a lot.
As you say, you're very new, so I'd look at getting a basic AP saddle like the new style Wintec, (Because I'm a huge advocate of this saddle for starters), because it fits most horses and offers enough support to your leg, especially considering you'll be riding in a completely different position. Another good AP saddle though is the Kieffer Munchen or the Kieffer Venezia or the Prestige GP.
Your hackamore will be fine to ride in, as another poster said already, but if you do want to get her more comfortable with having a bit in her mouth, I suggest something soft like a Nathe snaffle, or something not too bulky with a double joint in it like an elliptical snaffle, a KK, or a french link. Possibly even something with a non stainless steel mouth piece, like sweet iron or copper, to encourage her to taste the bit, rather than reject it outright.