Okay, I'm not an expert, so I firmly stand by none of what I'm about to say. I'm guessing that you might be like me in that you have a hard time keeping your fat-weight up – I'm 5'5" and I didn't get above 100 lbs until I was almost 20. And, coincidentally, I was about your age when I started working out to gain weight. I also started estrogen around the same age you're planning to. So, again, I'm not an expert, but I can share some of my experiences and maybe they'll be somewhat helpful.
You'll probably want to watch out for exercises that build muscle bulk until you start anti-androgens. What little bulk I did gain didn't last long after I started HRT, but that could also be due to the fact that I pretty much stopped working out. ^_^;; Be especially cautious of bulking up your
trapezius muscles 🔗 [Link: garagescapes.com/trapezius.jpg/], where your neck and shoulders meet. Trapezii like
this 🔗 [Link: bodybuilding.com/fun/trapscc.jpg/] are neither flattering nor easy to hide. Another thing it'd be good to keep in mind is, as I said before, body fat. Too little body fat and no bewbies for you. I was around 110 by the time I started HRT and was quite lean, but I still got up to an A cup more or less (more less than more). It has since increased to a B as I've slowly lost my superhuman ability not to gain weight. How that works out for you, however, will be pretty specific to your body. And as for strength, I didn't notice any big difference in how quickly it dropped off compared to the other times I'd given up my workout routine. Nor have I noticed a big difference in my strength when out of shape. You will not be suddenly unable to open heavy doors by yourself.

I hope some of this is helpful.