I do seamstressing/costume making and designing-and I think that if you are planning on losing a good deal of weight that would make you go down more than one size, that you should put off buying the dress. It is better to buy one later that fits-and only needs hemmed, etc. than to buy one now that the seamstress has to basically rip apart and completely sew back together. (Which is what has to be done...) Taking in a formal gown isn't just nipping in the waist...you literally have to slowly rip out every seam of most of the gown, and trim it all down (this involves lots of math...) and then sew the entire thing back together-not an easy task, especially when you don't have the original pattern that the original maker of the dress used when they made it the first time.
This is why alterations are so expensive.
If I took apart a simple Hanes t-shirt, it is actually at least 5 pieces of fabric. Imagine what a wedding dress is-with gathers, darts, embroidery, etc.
If you are in the 150's and planning on 120, I wouldn't shop until you are about 130.