You have different needs from a nursing bra at different points in breastfeeding.  

You should buy your nursing bras when you need them. 
 

• Comfort:  very important, as you will be tired and will want to live in comfortable clothes for a few days or weeks immediately after the birth.  Most women find they need to wear a bra because they leak breastmilk in the early weeks, so you need a bra to hold the nursing pads in place.  Many women find they prefer to sleep in a bra in the early days post-partum.  This depends on you and your comfort.  There is no substantial evidence to show that wearing a bra helps with breast shape or sagging later on, so don’t let that be a factor as to how much you wear a bra.  Use your personal comfort as a guide.  Some women are more comfortable with a bra on and some prefer no bra.
• Stretch:  Most women find that their breasts change in size from hour to hour in the initial weeks post-partum.  For this reason you will want a bra with a lot of stretch to accommodate this ever-changing breast size.
• Fabric:  a micro-fibre fabric is most comfortable as it has a tremendous amount of stretch while still retaining its shape with the added benefit of wicking dampness away from the body.  This makes it more comfortable and a better investment.
• When to buy:  you’ll want to buy at least 2 bras for use in the first few weeks post-partum in your 8^th or 9^th month of pregnancy.  You’ll want to follow these guidelines and buy a bra that has extra room in the cups but can be slightly snug around the ribcage.  Most women find their ribcage size decreases as soon as they birth the baby and their breasts increase.  Having said that, if you are buying bras earlier during in your pregnancy why not buy nursing bras.  There is a strong chance you will wear it at some point after the baby is born.  If you buy regular bras or maternity bras during your pregnancy you will probably never wear them again, until you next pregnancy at least. 
• My fav:   Medela Comfort Bra - this is the most comfortable bra you’ll ever put on.

• Support:  this is very different for every woman.  Try the bra on as many manufacturers add in support in ways you can’t tell from looking at the bra.
• Softcup vs. underwire:  during breastfeeding softcups are recommended but, I understand that if you are used to wearing underwires they can be enticing.  The reason for the softcup recommendation is that underwires can pinch the underside of the breast and cause plugged ducts which can lead to Mastitis.  If you feel the need for underwires try wearing one for only a few hours at first.  If you are not developing any plugged ducts then try it for a little longer.  Just make sure that you are always checking your breasts for plugged ducts.  Also, underwires can make it hard for the entire flap to fold down for breastfeeding.  So be sure to check this when you are trying on the bra.
• Stretch:  depending on your body you will probably still want a bra with some stretch as your breasts are still changing in size somewhat throughout the day.
• When to buy:  buy bras for this time period when you’re ready for the bra.  A bra bought during pregnancy might fit after the birth but it might not fit. 
• Invest:  invest in different bras for different occasions and different outfits.  Nursing bras, like maternity clothes and breastfeeding clothes, are improving dramatically everyday, so do some research and look around.  Amazing what you might stumble upon.
• My favs:   Medela Seamless Nursing Bra & Glamourmom (top with nursing bra)

• My fav:  Medela Seamless Full Fit Nursing Bra - seamless & supportive with sizes up to 42H