A while back REI had their brand duffels on sale for some % off, I picked up a few for just this application....but the final decision on how many and what size needs to be made based on specifically what is being packed and vehicle constraints
Everest also makes a couple of mildly mitigated bloody great sack duffles that are in the 12-15 gallon range. For a budget brand, they get surprisingly good reviews.
As far as what need what:
Religica and heirlooms, electronics, documents, etc. all need hard case protection. Food and tools, because of their weight, are also going in a hard case. Clothes, toys, diapers, and camping gear can go in duffle bags or suitcases. For the most part, they can go on the second layer of cargo.