I can't stress enough to find another GP even if they have nothing to do with prescribing your medications. I've had to deal with a GP that insisted that no additional care needed to be provided for transgender individuals, she was a nice doctor, but she was very wrong and fortunately, I knew just enough to know what she was saying wasn't correct. At the time I was locked into her because of insurance issues, but as soon as I had new insurance I dropped her. My current GP may not have much experience with transgender individuals but he is more than willing to learn, in fact, he took a day to attend a seminar in the twin cities when he took me on... that alone speaks volumes for the kind of doctor he is.
As already mentioned a local LGBT group could help, there is a database on line of LGBT friendly providers, but it's far from complete so it may or may not be helpful.
Therapist, you will need one and not all are created equal, don't be afraid to start a new search if you don't connect with them, they along with your support group are going to be vital.
Next, find a good voice coach that specializes in male to female speech patterns, you are probably pretty far ahead of the game being that you already have a trained voice, so it might be more about training new speech patterns into your subconscious then your vocal folds. I don't think it unreasonable to want to be able to sing afterwards, I've seen more than one Youtube video of transgender females who sing.
As to the rest, it is the same genetic crap shoot as the first time you went through puberty, everyone is different, you are going to have parts you love and parts you would love to change. Myself, in 8 months of HRT I am a very full 38B cup, waste is 31" depending on the day, and my hips measure 45"... I'm pretty happy with things so far, my face is another matter completely, but could be worse I mean I am 45. You have a lot of pluses with your age, the younger you start the more female you will look.
What to expect that everyone seems to go through? Your nipples will hurt for a good 6 months, but your skin will come soft and smoother, body hair will get less but will never completely go away (even CIS women have some body hair), keep an eye on your scalp though if you notice an increase in shedding talk to your doctor quickly, you don't want to go to lose half of your hair volume in about six months. Some people experience a change in memory, you will sweat less, your skin could be dryer or turn oily, the list goes on. Wish I could help more, but like I said it's a bit of a genetic crapshoot.