First welcome to BC. We are happy you choose to join our community. I removed you email so that you are not flooded with spam.
The internet issue is fairly straight froward.
Pick a WiFi router, less than a $100
A hub (4 port or larger)$40 to $100
Cat5 to reach your station (2-20' 2-10') $50 to $100
Print server (optional) $70±
NAS (Networked Attached Storage) less then $300 example
The protocol will be TCP/IP. The standard.
I prefer cable but DSL will be fine. You will pay twice as much a month (or more) for being a busniess. I recommended the NAS instead of a server so you would not have the expensce of a server. Divide the drive up and assign password protection to each folder. I assume you will be offering storage to the customers? 400 sq ft is a tiny space. 20x20 feet. Set the router on a shelf above the computer stations and reach should not be an issue. I would be very cause offering storage to your customers. All kind of sticky issues could arise for securing the files, to storing illegal software on your machine. Most users will probabily have online storage anyways.
How you secure you LAN is up to your, but I would not have the store computers on the same internet connection as the public LAN. You leave store buniess open to everyone. Put the store computers (cash reistes, work computes) on a seperate router. I woold make it wired. Have the 5 work stations and the public WiFi on the second router. If you add a full server instead of a NAS you can add $1000 to the cost plus learninig how to use the server software. My opinions were based on doing it cheaply, right? For your work stations learn on of the live versions of Linux (DSL, Puppy, knoppix, Unbuntu,..). This will run from RAM and reduce the time reinstalling windows because of malware and malious clients.
It is pretty simple actually.