I'll start with the easy one first since my wife again rehashed her 'Trust' & 'Betrayal' issues with me this weekend. You need to tell your fiancee ASAP. Oh, I know finding the right time is hard. As a former boss of mine once told me. "There is never a good time" (A real PHB. He also said "My door is always open, just make an appointment")
I also sure would not start anything transition related, other than therapy and seeking out a TG support group, before telling her. BTW-if her sticking around is an important go-forward consideration... Choose wisely young Skywalker. The odds are not in your favor.
My wife, who knew of me being a TG from about day 1 some 30 years ago and was mostly supportive and VERY understanding of the issue, has her doubts about sticking around. Her rational "I like men! I married a man, not a woman. If I wanted to marry a woman, I had my chance to". BTW - she is 25+ years post op.
There are things you can do transition related that take lots of time and money. Voice training, facial hair removal, even HRT (remember YMMV. But that is likely months away before you get cleared anyway). Oh.. and clothes! And makeup. And lots of time practicing your skills and developing a style that works for you. Hair, maybe, if you got it and can let it grow out.