If you have an MDE app and account for WDW, you could get away without a MB or MB+, but a MB makes things easier (in our opinion). You can buy one before you go (I suggest you would never require more than one, unless you want different colors/designs), or wait until you arrive. If you're staying on WDW property, there's usually a slight discount when you pre-purchase, but the overall selection might be better once you get to Disney. If you have a Disney reservation and you purchase through the app, your band should automatically be linked for you - or you can link your own. Not sure if this helps - I might have made it more confusing!
If it were us, we'd pre-purchase one MB for each of us, and use those during the WDW trip. It makes things much simpler since you can use it as your ticket, your room key, your G+/LL/ILL scan, your credit card for purchases, etc. Enjoy your trip!