I would look at a cloud hosted VOIP PBX. Hosted so the infrastructure can be dealt with off site, and all you have to do is configure the users, call routes, hold music etc..

Im not sure where you are in the world, so cant suggest some cloud PBX providers, but there are loads of them.

For the handsets we use Yealink who have a good selection of wireless DECT handsets, that run off a wireless DECT POE base station. Yealink also make wired handsets that are POE.