Heyo

My mom was a Wedding Gown Alterationist during my impressionable years, and I've gotta say: that Woman could work WONDERS! If you find the dress of your dreams, I'd say talk with the alterationist at the store and ask how many sizes down you could take the dress without it messing with the beadwork, overall design of the dress, and the neckline/armholes.
You'll want to be aware: if you want sleeves on your dress, I'd wait as LONG as possible until you're closer to your goal and date. If you think about it: if you look at a 3X t-shirt vs. a Medium t-shirt, The arm holes are MUCH different sizes. And, when you get into the "X's", they cut the arms wider as well as deeper to accommodate the lower armpit.
If you're going to go for a strapped or strapless dress, you'll have little to no issue with this, though!
If you went to a wedding dress designer to custom-make a dress for you, s/he would be able to make muslin mock-ups of dresses for you to try on, then make a dress fairly last minute. (this is easiest if it's a simple design with little to no beading). They would also be accommodating to your weight-loss journey! (and if you requested it, you could ask to keep the muslin mock-up dresses afterwards so you could see your journey that way, too! Nifty!!)
My last idea, though this is the most risky: Is to wait for a Wedding Expo. My girlfriend bought a $4,000 dress for $850 AND got a free Veil!--Brand New dress, not even a Sampler! It fits her beautifully. She bought it in February and she's getting married in August.
Congrats on the Engagement!! I know you're going to look gorgeous!