There are routers with USB support, but as you found they are expensive. Also, I would not recommend to get a router that does it all, after all, big companies separate the routing and switching, and there is a good reason for it. The more your router is doing, the more likely that it is going to fail, and when it does what happens???
efc idea is good because it separates your switching from the router, however you only have 2 wired devices, so it won't do much. You can skip the switch in your case (I use the router as a switch, but I have a switch to have LAN access in case my router dies, they always DIE)
As for the wireless router, I am picky. I hate to reconfigure routers every 6 months. I kill Lynksis really easy. The router that has lasted me the most is a Netgear RangeMax. That thing has 2 years, which is a lot for me. I had had to replace so many routers that I had considered a Cisco SMB at one time (300$)
For the NAS, there are lots of enclosures and cheap NAS that use cat5. I would use that instead of USB to the router. The main reason is that you can plug the NAS in any network and not just one with USB.
Another thing is that I use my computer as the network storage as well. So I don't really need a NAS because I always have my PC on. I mainly access my files from my PC, so I preffer to have direct access. I can still access the files from the other PCs in the house, and my sister can listen to my music with no problem.
So, the question is..do you really need a NAS? with 2 PCs, it seems to be overkill.
Get a large hard drive (or a couple and do RAID in your PC), and an external HDD for backup.
I work on a company with 11 stores, and 11 server, and I still cannot really find the use for a NAS to be justified in our enviroment (I do think it is great for other configurations thought)