selecting an adult working prospect

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Hi my name is Peter and Im from Australia I own two beautiful amstaffs that my wife my daugther and myself love dearly .I have now become interested in training amstaffs for schutzhund and family protection and have had my dogs tested but both have low drives and are not really suitable.I do helper work for a breeder of shepherds and malinois but have never had the chance to work any of the bull breeds.I now have a kennel available to me and would like to fill it with an adult amstaff prospect but I dont really know how to test and what to look for in a young adult amstaff prospect and what the major difference between working them to the common working breeds are.If anyone has any info on this I would love to hear it and if anyone knows of any working line dogs that are floating around in Australia I would love to hear of them to.
thanks heaps Peter