I don’t recommend buying a gown that is too small with the intention of letting it out.
There is a tendency to see where the old stitches were and may damage the fabric. Especially on delicate fabrics like satin and silk.
Sheer fabrics overlays like organza, tulle and chiffon, usually have been sewn with French seams and therefore have no seam allowance to let out, even if the fabric underneath does.
The most important part to be well-fitting is the bust area, as other areas are easier to adjust.
Almost any dress can be take in, but most can not be let out.