If you're saying "probably would end up doing some agility" I would make sure on that, and to what extent, before looking at a working-line dog. You wouldn't necessarily need to buy from a show breeder if you would like to show, but if you do get one from a breeder who shows they could mentor you in that aspect as well; if you get a working line dog you may need to keep conformation in mind when considering who and from what dogs to get a pup, and even then don't necessarily expect to get into big-time shows (which would be expensive to do anyways), but you could probably put some points on the dog at the very least.
Not all breeders of GSDs who show deserve the rep that they get: there are actually several breeders who do conformation showing and also breed for a more moderate look and also personally title their dogs in obedience, agility, even work them in things such as search and rescue or tracking. They also produce dogs who can perform. The key is finding an excellent breeder -- their dogs aren't necessarily any more cost-wise than others. You may very well not be able to title them in a bite sport (though I think some have produced dogs not too long ago who titled), and they certainly wouldn't qualify for police K9 work, but you didn't mention wanting to do any bite sports and the dogs who quite often are candidates for K9 work aren't going to a pet home anyways. This, however, doesn't mean that these dogs won't step up if called to protect, but neither does getting one from working lines (some of which aren't as much working lines as people are lead to believe) -- that comes down to more than what their type is. Similarly, working lines aren't necessarily healthier or sturdier than show lines, they're actually about matched up there when comparing good or excellent breeder to good or excellent breeder. The quality of breeder matters the most.
On a final note, the key to handling powerful dogs is building a good relationship and training the dog, which really often go hand-in-hand. What people call show line Shepherds are often more known for their obedience (i.e. titles and such) than the working lines which can be more independent-minded or require you to earn their respect first -- the reasons these dogs have been and are being bred requires more decision-making on their part (particularly the Czechs) whereas the show lines have been bred more for companionship in mind. Again, some of the best show line breeders breed for more than just show and companionship, but companionship is higher on their priorities than is often the case for the working line breeders (though the sport breeders I've had the most experience with take the cake as far as not keeping companionship in mind).
