Bella's wedding gown would be a typical early 1900's design, and there are some ready made gowns that would fit the bill. If you want something totally Bella, check with your dressmaker or find a vintage store and wear real vintage.
Music for a Twilight wedding would be Pachelbel’s Canon prior before the ceremony to be followed by Wagner’s traditional wedding march. Adding a piano to the mix for Edward would be a nice touch.
For a fun photo try this one courtesy of Meg Simone Photography, just in case someone wants to take a 'bite' out of you.
While many Twilight brides lean toward using a combination of red and white tulips from the Twilight cover, Bella's bouquet was actually all white. It contained orange blossoms, freesia, and roses. No red in sight. No tulips. This is good news for the budget concious bride because tulips don't hold up well and are season specific. An archway covered with white flowers and ribbon was the site of the ceremony in Twilight. While this may send budget-minded brides running, try an alternative. Start by using all white for the chairs and covers at the ceremony. Add a white aisle runner (an inexpensive rental, or buy one from a former bride on eBay) and use inexpensive silk petals to line the aisle. If you want to got the DIY route, make some red and pink topiaries in white pots for maximum impact at minimal cost if you do them using silk carnations.
You may also want to consider draping white tulle to form the archway or as an accent. All white glam is the way to go for this wedding.
For a fun photo op, consider finding some vampire fangs and hamming it up for your photographer.