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Most cost and expense with cameras is the wiring and labor to install. The trick is finding good locations with the right vantage point, then, getting power and data wiring to those locations. Those costs far exceed the cameras and recorders.
I dislike, and do not recommend Alarm.com. Their systems are sold through a network of local installers. If you call local security companies, you are likely to get pitched Alarm.com. Beware that if you buy one of these systems, they lock it down so that only that installer can do maintenance/upgrades. No other companies can work on it. Also, they make you pay an ongoing subscription, despite the fact that your recorder has local hard drives. If you cancel the subscription, key features like motion detection quit working.
I have had good experience with Ubiquiti. They are a system where you buy the hardware once, and you own it with no subscription required. You can hire any technician of your choice to do maintenance & upgrades.
To your question of who monitors the cameras: one of the trustees would likely be responsible for reviewing the cameras whenever there is an incident. A few high priced security companies will do the footage review for you and send back a report. Get ready to pay $$$ for that (eg Kastle Systems). As mentioned above, having the motion detection properly configured is crucial. If you have a 4 hour time window when the crime might have occurred, and no motion detection, you have to watch the whole 4 hours. But with motion detection, you can just skip straight between each time a person / car passed, and breeze through the footage review much faster.
Where to place the cameras: I agree, #1 priority is entrances and exits. If you have budget for more coverage, then I would prioritize, in this order:
#2: views of the green space/ common area, and any other areas with possible premises liability.
#3: views of any critical infrastructure or likely theft targets. Eg. do you have a storage shed with expensive maintenance tools?
#4: views of major intersections within your property.
Will they provide useful images? With such a large property, you are unlikely to have a camera view of the criminal act itself. With cameras just at entrances and exits, what you will get is a picture of everyone who came/went around the time. Now you have to filter that list, eliminate people you recognize as residents, to narrow down to the suspect. Even then, you probably won't have a clear view of their face.
One thing that might make the footage more useful, especially if your criminals are coming/going via car, is a special camera called an LPR - License Plate Reader: https://store.ui.com/us/en/category/cameras-bullet/products/uvc-ai-lpr - it will automatically log all the license plates coming and going.