Roofs and receivers have limited load capacity. You don't want to over stress you car when you really need the car to work well.

I agree with Tjin. Roof racks are fine for ski's, but for any serious packing it should be inside. I'm reminded of the insurance commercial when some guy drives into his garage with two bikes upright on the roof of his car

I like the idea of a trailer hitch bike-rack though. As it is if things go south due to a fire I will lose my bike, now hanging from the garage rafters. A bike rack would allow me to take it along and give me a transpo option when gas could be tight.

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.

Good plan. I may look for a low crate type container to form a layer under the duffel bags. There's room vertically for another shallow layer and below the duffels would be better as you point out. Thanks.

Roofs and receivers have limited load capacity.

It depends on both the "car" and the receiver. If it's a pick-up or SUV with a body on frame and a 2" (50mm) receiver, the limiting factor will be the cargo rack.

more to maintain it and in tight spots a small car is a lot more handier.