I'm not sure what everybody is making such a big deal about. There are a lot of missing steps, but old laptops are free, and proof of this concept has existed on the internet, including DIY videos, for some time.
The question is scalability, in which case USB is pretty much just not an option for more than like 3 devices, but PoE is a reasonable thought. You may need to purchase or build individual controller boards for each camera, but that (and adapters, and an old 48 port gigabit switch, and a central server/NVR) will still, indeed, be cheaper than purchasing enclosed, off-the-shelf IP cameras; which would still require not only a substantial length of Cat5e cable, but likely also a specific brand of NVR and/or software subscription.
I unfortunately do not have all of your answers, but if you've taken a crack at it at this point, I would be interested in hearing about your results.