Smallish office - do we need a standalone HW firewall?

There are quite a few considerations when you deploy wireless in an office environment. This is why site surveys are typically required or some sort of RF predictive/planning tool. In office environments, you might run into issues with the actual construction of the building such as any concrete/cinder block walls, metal studs, etc, etc. Also in business environments, you're probably competing with other businesses running their own wireless networks. Layer in the requirement to run VoIP wirelessly, you have some challenging requirements.

Use of SOHO wireless devices should be immediately crossed off your list. With regards to your possible candidates, be aware that Meraki was acquired by Cisco. So if you're having issues with how they handle licensing, you'll be in for a shock when they get assimulated. Ubiquity is a unique proposition but based on some information I've seen on how the internals are in their devices, I'm not that impressed with the quality.

An option to consider is Aruba's (Dell PowerConnect W--OEM'd from Aruba) Instant APs. These APs provide many of the functionality of a full wireless controller solution without the cost. The first IAP set up on the network becomes the virtual controller for the entire network. Each subsequent IAP you add just joins the network and is managed by this virtual controller. If the virtual controller fails off the network, the remainder IAPs go through an election process to promote itself as the new virtual controller. The IAPs provide the ability to do band steering, spectrum analysis (depending on the IAP you are using), RF management, and have a built in firewall. The nice part about this setup is that there are no licenses to deal with.

Another option is to use a SonicWall firewall and then use their SonicPoints AP as every SonicWall firewall can act as a wireless controller