When it comes to lingerie, you can spend $100's but the most important things to remember is your comfort and will it show through your dress;
I highly recommend being fitted for any lingerie by a professional. You will be very surprised on what bra size you really should be wearing.
Also keep in mind your body shape. If you have tummy, consider wearing a body suit.
Lacking in the cleavage department? A push up bra can help in this area.
When to purchase your lingerie? Best time is to purchase any lingerie is just before the final fitting of your gown. This will allow your designer to make any changes and ensure the gown fits you perfectly.
Stockings? if you decide to wear stockings or pantyhose make sure you have a spare pair and or invest in a extremely durable pair to reduce the risk of any laddering.