Hmm, let's see now... I understand that you don't feel secure at the moment and I do agree that you're not just entitled, but obliged to find ways of rectifying that. However, to my mind, protection consists of what actually helps you *be* safer, not what you imagine will make you *feel* safer. Just thinking of how suitable a protection trained dog would be for you, the first thoughts that come to my mind are:
You don't want to train a dog or think you can't train a dog on account of your disability. This also means that you're unwilling or unable to teach the dog to obey you and subsume itself to your authority, that you cannot maintain the standard of training it has and adapt what it knows to your particular needs and that you will not be able to control it. And an uncontrolled biting dog is a far bigger liability, heck, make that a far bigger direct danger to you and your family, than most crooks.
Do you enjoy and understand dogs? Are you willing to work with them? Dogs are above all, living, breathing animals which do make demands on you. I've seen quite a few people who aren't really fond of dogs but keep some anyway for their supposed security value. It generally doesn't work too well. People who do enjoy dogs and who do wish to take time and effort to work with them generally end up with much more useful dogs, even if they're not spending huge amounts of time on day-to-day training.
Do you have an idea of what kind of protection dog you'd want? Since you mentioned a GSD off the bat, I'm getting the impression that you have in mind a Rin-Tin-Tin like dog that would take on crooks effortlessly. In reality, there's a wide range of applications for dogs in the business of making us safer. I've seen excellent dogs who'd silently bring the existence of anything unusual to the attention of their owner (do not underestimate how useful this can be). I've seen excellent dogs that would make nearly any thief alive sorry to have wound up in striking range. And I've seen excellent dogs in between. They're all excellent dogs because in each case, their owners gave thought to exactly what it was that they wanted and needed, then selected and trained the dogs for that purpose.
So the questions are: are you absolutely clear in your mind as to what you expect a 'family protection' dog to be able to do? Are those expectations self-consistent and realistic? Do they meet your needs?
Fixed income. While this is not in itself a disqualifier, it says to me that you'd easily find yourself in trouble with a dog. A good dog has its price and this isn't a product of greedy breeders and sellers: it reflects the considerable cost of selecting, raising and training and the more involved its training, the higher the cost. While cost isn't everything, a cut-price 'protection' dog is nearly-guaranteed disaster.
And once you've purchased a large, energetic dog, what of its needs? Are you genuinely able to meet the cost of housing it properly, feeding it high quality food, vet bills (including emergency ones)? No? Don't get a dog.
Believe me, the alternatives of various alarms, more secure windows, good lighting, etc help considerably. You can find some pretty good books on considerations for personal security in just about any local library -- a free and excellent resource. Or else order one from Amazon.com. Even if it's as 'simple' as a padlock, all security systems require some expenditure of time, effort and money to put in place, due diligence to apply and maintain and require an appreciation of their limitations. A realistic, non-emotional assessment of your risk also helps. Putting in place a system that you cannot properly apply for whatever reason is often worse than none at all.
It's not about what worked for Mr. Brown in the other town or about the stories of amazing derring-do. It's about what will work for you. I'd seriously reconsider the dog if I were you.
Sincerely,
Dei.
The plural of anecdote is not data.
-- Stephen Budiansky.