Skifast, I do this kind of thing both for my Church and for a large number of Marine Corps sites here in the states. You need to start with a threat/vulnerability assessment and a site survey and work the problem from there. I have noticed that in the wake of recent Church/school shootings everyone is concerned about active shooters. You must have a plan for an active shooter but there are a lot of things that can be done ahead of time to prevent that incident or at least set you up for success if it happens. That is the worst case scenario but there are a lot of threats to a Church that are much more likely to happen so you have to make sure that your plan is an all hazards approach and not just dealing with one or two worst case scenarios.
Use the resources that are available to you or offered to you by others but look at your Church's specific situation rather than cutting and pasting. This is where a site assessment is key. Every Church is laid out differently with different access points and different types of construction and what works for one Church may or may not apply to another. You really have to red cell a location if you want a true assessment of your situation. At work, before I even go in an talk to a site commander I do some surveillance and see what I can pick up about the location. I figure out what I would do if I wanted to harrass the site. I figure out what I would do if I wanted to make a statement. I figure out what I would do to inflict maximum damage/casualties. You have to do the same for you Church.
For instance: What if you were divorced from your wife, she wouldn't let you see your kids, and the best way to get them back was to take them while she is in church. How would you do that? Walk up to the nursery and check them out? Take them from the Church playground? Now how do you stop that?
What if you were mad at the Church? Maybe you were a deacon who got caught in an affair and felt you were publicly humiliated by the pastor when you were asked to step down. Your point is to inflict as much damage as possible. How do you do it? How do you stop it or mitigate the damage that can be done? A lot of this has to do with the layout of your building. Look at the front doors to your church; are they knobs or are that big brass D shaped handles. If they are completely enclosed in any way, the first thing I do is chain that entrance shut...maximum casualties as people can't get out the main exit. How do you mitigate that?
Just some thoughts. Red cell any plan that you come up with and see what you would do if you were the bad guy stealing the offering, being the active shooter, stealing a kid, or breaking into cars in the parking lot.