I'm curious what these old peripherals are that you need old PCs to support.
A friend of mine claims his PC based firewall does something superior to my Linksys router with alternative firmware. Sure it's more powerful but it also uses more power, generates more heat, and takes up more space and time managing it. My router is small, silent, and I never have to touch it. It routes my packets and protects my network. And I don't see the point of having and old PC for web browsing if you have a better computer at your disposal. It's either the primary computer for some user in the house, provides a specific purpose that can not be provided by the primary computer of some user in the house, or it's trash.
Using a PC as a NAS is definitely preferable, in my opinion, to a consumer NAS. Those things are invariably featureless and fail if you look at them funny. But if there's less than a handful of PCs on the network you might as well just throw a huge f'ing hard drive in one of the workstations and share it to the network. That's all your "PC NAS" is anyway.
In my house:
Primary workstation shared by my wife and I, and an occasional overnight guest
Also serves as file server, print server, and gaming PC
Printer/Scanner attached by USB
Roughly 2TB of storage
TV PC for video playback and rare gaming sessions, attached to TV
Loads files from workstation
Old laptop on a custom shelf in front of the treadmill for video playback
Loads files from workstation
Router/Access point with DDWRT (need to look into other options, but it works so why bother)
We each have a smart phone that talks on our wireless network.
I have a work laptop that I bring home for gaming and general surfing (and occasional work, hate to admit).
What do you do on your home network that I can't do? Honestly - I want to know. I have three computers and a router for two people.