I don't know anything about service dogs I'm afraid, but there are several very knowledgeable people here on that one, so hopefully one of them will chip in :)
how can you tell that you're able to have a dog?.
One option, borrow someone else's dog for a couple of weeks - if you maybe know someone with a dog who's going on holiday. Or go help out at a rescue centre for a bit - it's not the same as having a dog in your care 24/7 but would give you some idea of some of the needs dogs have and whether you can meet them.
What responsibilities does it entail to have a dog?
Taking on a dog you need to think about being committed to that dog for the next fifteen years (some breeds live longer than others, but whatever you take on, please be prepared to take it on for life and with the understanding that an old dog has different needs and can be a very different experience than a young dog.
Basic responsibilities - feeding, walking, cleaning up its shit, taking care of it's health, training. All dogs needs exercise and stimulation, some more than others, every day, whatever the weather, you can't just have lots of can't be arsed today off days with them. A dog lacking in exercise and stimulation is likely to end up with behavioural issues because it will try and find its own entertainment. Think about where you're going to exercise your dog, do you have safe places to walk it, do you have a secure yard etc
Healthcare - work out where your nearest regular vet is and where your nearest emergency vet is. Work out how you get a healthy dog there for regular stuff. Then work out how to get a sick dog to the emergency vets in the middle of the night/on a weekend/holidays.
Depending on whether you go for the puppy option or rescue option, training needs vary. Puppies are a LOT of work, and need a lot of patience and consistency from you. Rescue dogs can come with all sorts of issues depending on their background and history. Some have no problems at all, others may come with their own trauma history and associated anxieties and problems.
You also need to think about your lifestyle. Do you work, are you likely to work, a job that's going to mean you're away from the dog for long periods of time? You need to factor in some kind of day care for the dog if you are. Do you like going away - factor in/figure out what you will.do with the dog when you do that.
Can PTSD keep a person from having/taking care of a dog?
I think that depends on what ways it mainly affects you. If your symptoms would prevent you exercising the dog, or being able to get food for it, meet its medical needs etc For me, having a dog helps because it makes me have some routine and makes me have to leave the house every day, which I am not always able to do for myself, but because I am committed to caring for the dog I do it - I can let myself down, but I won't allow myself to let the dog or the rescue down! It's also in some ways given me some more insight to some of my own problems (the dog I foster currently has a lot of anxiety issues) and it also helps me to practice patience and tolerance.
If you're low-income is it best to not have a dog?
This is a tricky one. Dogs are expensive. You have to factor in food, grooming (for some breeds), vets bills (both regular and unexpected) - insurance will cover a lot of things, but obviously you have to pay for insurance too. And all those things you have to think about for the rest of the dogs life. Having said that, I know many people on low incomes who give very very good lives to their pets, and I owned a dog while I was on a low income and she was very well cared for - I would always go without before letting my pets go without.
Are there really untrainable dog breeds out there?
Some breeds tend to be easier to train than others. Some breeds are more intelligent than others. Some breeds are more suited to different types of lifestyle. I'd say ALWAYS research the breed before taking a dog on.
That's what I can think for now. :)
But just to add, having a dog is hugely rewarding too :D
