Looking for a guard dog

We'll be moving to a new place soon that we like very much, but security is a bit of a concern. The criminals have lately also become rather brazen, as things like armed reaction and electric fences hardly deter them. An electric fence is actually only useful as an early warning system, but the new property has pre-cast walling and this would require a full height fence, not just something added to the top of the wall. With the perimeter being rather long (it's a half-acre property) installing such a fence will be prohibitive.

For a while now I've been considering getting a big intimidating dog. These are the only effective deterrent lately, but these also come with problems:
1. How are they with kids?
2. How smart are they?
3. How trainable/obedient are they?

I've really been thinking of getting an Alsatian as they're smart, naturally trained to defend and very very trainable. But statistically it's been proven that Alsatians are responsible for the second most accounts of dog attacks that end up in hospital. The statistic doesn't state whether these are kids or grownups (or both), so it may mean that they're either very effective guard dogs or something unpredictable.

My primary reason for a trainable dog is to teach it to only accept food from me and not to eat just anything that is thrown over the wall as dog poisoning is very common lately (I know of four incidents in the last month).

It's unlikely to get consensus here, but I'm hoping for some valid opinions on this approach.