Yeah, deerhounds are generally quiet and not much of a guard dog beyond their large size putting people off messing with them. Your bro probably knows this already but these big dogs need a real good diet, with lots of chicken frames and chicken necks and a good dry food to support their spectacular growth rate. The chicken frames and necks give them natural calcium and phosphorous for their bones and also protein and fat. Calcium supplements are BAD and can cause abnormal bone growth.
This dog's shown guarding instincts from since when he was little. I don't mind it and I know when the Mrs goes walking with him she's safe as he'd tear your face off if you had a go at her.
I just warn people that want to pat him, to let him suss them out first rather than just shoving their hand in his face. He tends to move away if that happens until he's satisfied they're OK, but I prefer to play it safe.
As for other dogs.....he's generally fine but is extremely dominant and given what wolfhounds were bred for I keep him away from other dogs in case they have a go at him. He's as strong as an ox and takes some controlling.