Two things come to mind; the legality of it, and the need for it. As for the legality of it; I wont speak on that, thats for you to look into. As for the need...is there trouble there? Has there been? I lived mostly in communities of 50,000 or less, and have never seen, nor even HEARD, of a church having a protective detail until a few years ago. Where is our society going?!?!?!

All that being said, here is how I feel it should be run; security guards, armed or otherwise, should be entirely separate from the congregation. They should neither be hired from, nor belong to, the congregation. This prevents any possible legal issues.
If they are armed, ENSURE they have had the correct training!!! I cannot stress this enough; someone who shoots a kid for spitting on a sidewalk, the whole church would suffer. Firearms come with daunting responsibility; do NOT take this likely.
Uniforms are an absolute must. IF there is an incident, it is easy for the local PD to pick out who they are. And, a visual presence can prevent most issues.
The congregation should be left alone-if someone carries in church, that is his/her business. Legal or not. Unless they are passing through metal detectors, or causing issues, there would be some legal issues with a search of a person, unless there is some sort of agreement beforehand. And, unless your security detail are police, or have had police training, they likely dont know how to search, and what violates an individuals rights, etc.
There are a lot of legal issues involved here-and, its best not to skirt them. You can put the whole congregation at risk. CCW plain clothed security isnt an option IMHO; there are too many variables. I also am not a fan of allowing members of the congregation to carry firearms, as you dont know the level of training of them-people panic in times of stress....

Sorry, but I simply cannot fathom going to a church that needs security...I dont think I would want to be part of that community, if I had a choice. No offense to the OP...but a place of peace shouldnt NEED this level of protection. It simply baffles the mind!!!