I've utilized Sonicwall firewalls (including the TZ 215) at several small businesses and they are great firewall appliances for the cost, and as stated above you can also purchase additional subscription services for additional features if you need, such as content filtering, if you choose you need those services.
However, in all honesty, I would not recommend purchasing the wireless TZ 215 for your situation. A single wireless point is not going to offer enough wireless coverage, and from my experience working with Sonicwall wireless AP configurations, it can be very tricky and somewhat limiting. I would instead suggest utilizing the Sonicwall as your default gateway router, firewall, etc. and purchase separate PoE wireless access points. Again, Sonicwall makes stand-alone access points, but these can't be individually configured and REQUIRE the Sonicwall firewall to actually access and configure the devices. This was enough for me to stop using them. Also given the cost, there are many other alternatives with more features.
I started looking around at other brands and options, and the two that really ended up good for us were EnGenius and Ubiquiti brand wireless access points. Both have a broad range of access points with varying features to meet your needs and your budget, and of course there are also many different "long range" options with increased performance antenna arrays to over greater overall coverage area.
Again, I've personally had great luck with the Ubiquiti UniFi Long Range WAP units, and given the price there is little which can compare. You can broadcast multiple SSIDs and operate with VLAN tagging, which is a feature many cheaper units just don't support. It's hard to know exactly what you need for your office, but I would recommend utilizing at least two APs, and that way in the even that something causes an outage with one AP the other is still up and running for your wireless network. I've personally tested the UniFi long Range WAP units with about twenty concurrent users connected and there really didn't seem to be any sort of trouble with that load, and they are rated for up to 30 users or more.