Thank you ladies so much for the comments! But it turns out, I won't be getting the dress. For now, at least.
I technically already have a dress for this party: the ink-dot dress from J.Crew. It's navy blue and white and really versatile: I've worn it already to my bachelorette party, rehearsal dinner, a birthday/grad party and will wear it again (with my Carolyn espadrilles, this link is the exact color of them) to a backyard engagement party this June.
When The Aussie spoke to his mother about the party and how I needed to get a dress and he wear a suit and we needed to get a gift (we thought this was a formal party, you see), his mom said that I can wear my "navy polka dot dress" (she likes it alot).
The point of the "Black and White" attire note was to make sure that the guest of honor - his godfather - stood out amongst the guests. I think he doesn't know that this party is being held.
But when you get an invitation to a party and it says "Black & White" attire, don't you think it's a formal one? As in the evening? Or early afternoon into early evening?
Well, not when it's 12:30pm in the afternoon. Sigh. I will keep an eye out for that dress - maybe I can try to get it on sale or at least, I won't be in that much of a rush to get it. That gives me a bit of time to lose some more weight that I gained from PTWS (post-traumatic wedding syndrome).