Since you are going to be running the PoS and other internal computers off of this network it makes it a bit more complicated to keep it secure. You need to provide two different networks, one internal and one external for the customers.
It is advisable to have a set of bridges provide connectivity to the restaurant from the pole into a wired switch. This link will be encrypted. From the wired switch, you can use two Linksys routers / AP's. Each AP will have it's own public IP from Comcast.
Each router will have it's own set of private IP's. It would be good to have them in different subnets, although this isn't required. It does help to keep things straight.
If you plan on having a wireless network for your internal (non-customer) network, it will need to be encrypted as well, which isn't a big deal. WPA-PSK with AES. Just make sure the PSK (Pre-shared Key) is really long (30+) characters. Most people can't easily remember 30 characters anyway, so I max out the PSK to 63 characters.
As far as enGenius equipment goes, I'm no longer a big fan. I used them in about 30 of my hotel jobs and, luckily for me, enGenius had a rash of bad equipment and I had to replace a lot of AP's. I actually just sent back 16 (yes, 16) AP's back to them with bad radios.
On the other hand, we use enGenius in our WISP and they work ok. Personally I think their failure rate is a bit high, but a lot of people use them so I can't say that I am right from my experience alone.