Everything you're wanting to do can be done. It's generally much easier with enterprise grade equipment, such as ubiquiti or microtik which are on the cheap side.

As far as the computer and FreeNas, unless that computer can run ECC ram(server grade) you might be better off turning that box into a pfSense router/firewall. FreeNas is designed for enterprise grade storage. If your looking for simple, easy storage you could much easier take that old computer and turn it into a Debian or CentOS caching server much easier than using FreeNas. You'd probably want to look into a gig our fiber NIC card, you can get used server ones pretty cheap on eBay still.

If you want to stick with your D-Link, you probably need to look into building or buying some type of waterproof box since it will be exposed to the elements, and you need to look into ddwrt or tomato.

But generally running an open Wi-Fi box isn't that hard. Most routers rented or sold by Comcast or Cox or the other ISPs broadcast a separate Wi-Fi network for their customers that anyone can connect to.

I'm pretty sure, you're ok if you run an open Wi-Fi. Of course you might get your equipment confiscated until they clear you, especially if you're doing something like running a TOR node or endpoint.