Working with that large amount of passengers is more daunting than you may realize. Being that it is a "couple years" before it will actually happen, there is no way to give an accurate possible price structure you will be looking at....too early.
You should have a meeting with everyone and decide the length of cruise, itinerary preferred, and per family budget to work within. After deciding that, begin your research.
Personally I would not rule out working with a TA on this at all. Their services are free and will eliminate your headaches i.e. collecting deposits, final payments, correct names for documentation, going over all the mirage' of questions that are certain to follow..... ARGH !!!
Best of luck in your reunion cruise !