From someone who is getting married next year...
I tried the on-line route and wasnt blown over by the quality. I have bought my dress from a shop and it will cost around £1,000. The dress is designed in the UK. The quality from overseas suppliers is very hit and miss and wonder where you will be getting your dresses from? - apologies if you have already mentioned this, but I have started typing....
I have a friend who started a business just like yours and it didnt do well. I think if you aim for high end customers with high end quality, you are more likely to do better. Most ladies want that extra special dress for their big day and are willing to pay a bit more for getting that quality, but that may mean your costs will get higher.
I hope it works out for you!