Cheapest option maybe, probably not easiest in the long run. All of the T4xS phones and some of the TxxG phones can be made wireless. There are actually two wireless USB adapters for those phones currently, the WF40 and the WF50. The WF50 requires a newer firmware than what 3CX currently supports but the WF40 works fine. You can of course use a wireless ethernet bridge with any IP phone as well. Keep in mind that 3CX will not know the phone is wireless so you'll either have to manually configure the wireless on the phone or make a custom template with your wireless settings and then provision over the wire first before relocating. Not ideal but they work fine after.
It does have STUN (some posts suggest it doesn't) , and I have several of the keys as extensions off other 3CX installs. It can also be configured via a GUI, which some Cisco sets cannot. They are getting a bit long in the tooth, and may be getting harder to get, other than used. Still think it is one of the more versatile, option packed, desk sets out there.
Ahh yes, the oldie but goodie. But definitely wouldn't be buying those phones for new considering their status in 3CX. And from the wireless perspective they still need to be manually touched for the wireless configuration.
If it were a whole office load, then no, it's too much work to configure, but for a one or two of, in that special location that requires wireless, they are hard to beat.