All of the costs outlined by QED will add up and do need to be considered.
But I imagine the cost of stock is going to be your biggest!
No point opening a shop if you cannot sell the products so do some homework about what other shops do, where they get their stock from, how much does it cost, are you going to have to pay for the stock up front?
Also very important is to consider, can you sell?
How much cash do you have access to and how much cash will it cost to set up the business? then work out how many months you can afford to keep the business going for without making a single sale? I would probably suggest that if you cannot afford to keep the business going for say 12 months it is not a runner. In saying that I know nothing about wedding dresses but just imagine it will take a few months to make your first sale and imagine you get paid in installments!
Basically concentrate on your cashflow!