Mark, your plan looks fine. The e4200 will be able to handle all the devices without at problem. It is the high end consumer model with quite a bit of horsepower. Using switches to add more end-points is the way to go, rather than looking for an expensive router with more ports. I have three switches in my house.
As David pointed out, some USB-connected hard-drives have had a problem with the latest e4200 firmware. I am sure this will be fixed in a future firmware update, but always have your data backed up.
If you want to go with a dedicated NAS, you can buy a BYOD model and extract the disks from all your USB enclosures and use them in the NAS. Just check that they are all SATA connectors inside. I would recommend a NAS from QNAP.
For the wireless connections you want to try and place the e4200 so that the signal covers the house nicely. I have an almost-dead spot in my house, and when I move the router the dead-spot moves to somewhere else. It is very annoying in this age of smart-phones where you expect to have coverage even in the bathroom.
If you want me to stop by and help, just say the word. It's easier to spot/prevent issues on location than via a forum. :-)
