If you have ghetto's in your area, go to the shelter or pounds. Lots of pitbulls and rottweilers there. They get taken away from all the mexicans and blacks. they have usually been mistreated so they are mean. However, aggresive dogs are put to sleep.
An alarm system doesn't need to be fed, go to the vet, have a bath, etc... Not as expensive as you think. You don't need to get the service, just install motion sensors and the alarm itself.