OP, yes you can wait until you get home or even do it mid trip if you want.
If you buy Memory Maker in advance, you will save the $30, and only pay $169. You just need to make sure to do so at least 3 days before the start of your trip. If you would rather hold off and wait until you see all the pictures to see if Memory Maker will be worth it for you, you can do that as well. You will need to pay the full price of $199, and doing so will still let you download any non-expired pictures in your account.
To break even on the pre-purchase price, you would need to want 11 downloads/prints. Only you know if that will be worth it for your family or not. You don't need to activate Memory Maker for this trip if you decide you don't like enough pictures to make it worth while, so you could buy it and hold onto it for another trip.