Hard to say, but I have questions. Is this your first dog/puppy? Have you had GSDs before? Any working breed before? Why this and not another puppy/dog? Several hundred dollars is pretty standard among reputable rescue groups, though it's much less than you'd pay a reputable breeder. Is that what concerns you?
If interested, I'd go observe the pup where it's currently (or supposedly) living and ask a bunch of questions --- how long have they had him? Where did they get him? What's his vet history and I'd ask to see those records (e.g., timely well puppy visit, vaccines administered, at the right time, not all at once, any other vet visits and, if so, for what, etc.), what have they done with the dog so far (e.g., formal ob training, house/manners training, etc). I'd also be interested in observing a bunch of stuff --- how does the dog respond to a stranger (you) coming in? If suspicious, how long does it take him to recover and become interested? what does his temperament appear to be like (if you've got some experience and know what you're doing, you can do your own temperament assessment to get a feel for the pup), how does the dog interact with the 'owners?" If you can, take him for a walk, off property, and observe how he does (e.g., good/no leash manners, reactive to other dogs/strangers?). What do the house and yard look like --- clean, littered with trash and poop?
What are they feeding and how often?
Then, if you decide to go forward, I'd make everything conditional on a successful well puppy visit with your own vet. I'd even schedule that visit for the next day, explaining to your vet why you want to set things up that way. You can always cancel if you don't take the pup.
A couple of final thoughts. First, good dogs/puppies can wind up in all sorts of unexpected locations. You never know, sometimes you can get very lucky and sometimes not so much. Second, even well bred puppies from reputable breeders can develop significant health issues because stuff happens. Third, in a risky situation like this, you need a clear idea of what you can tolerate (at a minimum), and what you're able and willing to do (w/re management and training) to end up with the kind of adult dog you desire, doing the things you both enjoy.