All breeds have their good members and their bad members. Unfortunately, it’s the bad members that get all the press and gain any breed a bad rap. Pitts, dobbies, rotties, and boxers – along with chows, akitas, and even mastiffs – all suffer this notoriety to some degree.
With any large breed – especially one bred to be a guard dog of any kind, or that has had any breeding for an attack or fighting instinct – good training is always a must. A well-behaved dog will put many fearful folks at ease, and dogs pick up on fear and nervousness, and will echo that feeling. Also, you need to be sure that you will be able to calmly handle the dog at all times, no matter what is going on around you. If you start to become anxious or afraid, your dog will pick up on that, and that’s when bites happen – to you, or those around you.
Last but not least, you need to objectively assess his “special” needs. Are they likely to flare up in the future? Are they likely to cause other health problems? Do they hinder him from getting the exercise he needs? Dobermans are mid to high energy dogs, and need a good deal of exercise in order to behave properly. Will you be able to afford his old age, when his pre-existing conditions may cause costly vet visits? Are those special needs going to require more immediate costly vet visits? Will you be able to, in all honesty, afford these visits and anything else he might require to make him comfortable without seriously straining your budget?
If you can answer an honest yes to the last question, and you are willing to commit to calm, positive training and patient rehabilitation of any previous-owner trauma, then I say go for it. But be absolutely certain before you take him on. It’s very, very unfair to the dog to “take him back” once you’ve invited him into your home, and it’s unfair to put yourself out, because you will most likely, albeit unconsciously, blame the dog for your misfortunes. Taking pity on rescues is all well and good, over extending yourself causes more issues than rescuing the dog solves.
Best of luck!