Melanie,
It's not an inevitability at all-it mostly depends on how much you weigh, and where that weight ends up going. If you exercise, and control how much weight is going to your hips specifically, you should be ok. Generally ~15~ pounds is all you should expect to gain. Any more than that, and you can begin to worry about developing celulite.
As for breasts, it really depends; I'm going to have to disagree with the other girls who have posted here. Unless you're incredibly lucky, you probably won't make it past a B cup (and for broad chested girls, that's really nothing). I've been on HRT for a year and a half now (I started at seventeen, too) and I'm still a AA. Everyone is different, and your results may not be typical-but if you're looking for a full, feminine chest, and aren't the luckiest woman on earth, I'd start looking into breast augmentation.