
I've done a lot of research on these years ago, and they are still on my wish-list. They are I believe still considered semi-domesticated because of their background.
Territoriality is a big consideration - these guys are bred to grow up with, live with 24/7 and protect their charges from large predators (typically wolves but they are used a fair bit now in Africa to protect goat herds from cheetahs). So they are highly territorial and I did read once or twice that sometimes, they can extend their perceived territory to regular walks.
Fiercely independant breed - they have to be as it's down to them to make the decisions about potential threats. But that said, raised properly, they should only use the minimal force necessary to see the threat off. By nature they are reserved and cautious of strangers but, they have been bred here for decades now and I think that side has eased off a bit. Certainly the ones I met when I was researching were lovely - not crazy friendly but lovely. Those were show dogs.
Whether they can be let off lead seems to be individual - I got a very thorough email from a breeder and she said that of the four adults she'd had at the time, two were always on and two were always off. Same upbringing for each IIRC. I suspect that with the independence, off-leading could be a challenge as they are apt to just make their own decisions.
Backyard breeders are becoming a problem - they are generally a very healthy breed and long lived for a giant (15yrs is average), but hip dysplasia has started to crop up in recent years.
My concern with a rescue would be socialisation and training thus far - especially if he's gone into rescue at 8 months old. Typical time for many dogs as it's the start of the teenage months, selective deafness and so on but in a giant dog with strong guarding instincts and a very powerful dog (and when they get moving, very fast too) a lack of training/socialisation previously could be an issue. Not to say it couldn't be overcome - I'm working with a 1yr old St Bernard atm with zero training and he's coming along beautifully :-) They can be outstanding, of course - you only need to google Haatchi and Owen (winners of Crufts 'Friends for Life' last year) to see how amazing a dog they can be :-)
Good in heat and cold - thick weatherproof coats designed to cope with extremes of temperature. I suspect big moulters, but if he's used to GSDs that shouldn't be an issue! They do need decent exercise - a couple of hours a day with mental stimulation too.
Security is important too - again not the same for every individual dog but generally speaking, they need good, strong fences and are deceptively agile. This is why I don't already have one - my fences are simply not strong or secure enough for such a dog, and I have young kids living either side of me. So I'll have to wait!