Hairspraybabe is spot on.
I too used to sell wedding dresses.
If you go to an actual shop tell them about your body likes/dislikes and what your wedding is going to be like they KNOW their stock and which designs accentuate/disguise certain areas of the body.
Certain styles look better on a fuller figure than a slim one (and of course also vice versa) it's also about body and face shape, skin tone (few people can actually wear pure white eg). It's almost impossible to tell without trying them on.
Several times I had brides struggling to find 'their' dress and was able to suggest a dress they had dismissed initially only for them to fall in love with it. I think it's largely because we don't wear that type of dress regularly so of course don't know what suits us.
I LOVED that job.
DEFINITELY only have a specialist seamstress do the alterations - in house are usually the best both for convenience AND because again they're familiar with the dresses in stock so will have worked on that style before. Also because the fabrics involved are delicate and easily ruined.
We had several panicked brides who'd gone to high street alteration places (type of place who normally replace zips/sort hems on school uniforms) where dresses were all but ruined and our seamstress rescued them. (The dresses AND the brides!).
You'll also be amazed at how much an experienced bridal seamstress can personalise a dress.
Good luck and have a wonderful day.