My response in a nut shell:
I am really happy you emailed me with these questions, and I am very sorry it has taken me this long to get back to you... Now, let me see if I can help!
With regard to this being a primary or part-time career, you CAN make this a primary career, but you really need to have consistent clientele to do that. The less you limit yourself, the more money you can make.. I have friends that do pet-sitting, board 'n trains, dog-walking, private clients, group classes, workshops, volunteer seminars, invisible fence training, etc. etc. Obviously the potential to make money is greater when you can offer more.
Personally, the rates I charge vary a lot depending on many factors (location, what's the issue, etc.) AND the more you invest in your business, the more overhead cost there is. As in any other occupation, this effects how much you charge.
Shelters can always use volunteers. Getting your feet wet there just for experience with lots of different dogs is a great idea, even if you don't have training experience.
As far as not having a dog... You need to get you a dog, girl! :) It isn't a requirement to have a dog to be a trainer.. I didn't when I started.. but you don't make a very good impression on clients when you tell them they are paying you to be an expert on dogs and you don't even have one. Again, it doesn't mean you can't do it... You wanna get around dogs as much as you can, though, especially if you don't have one of your own because of life constraints.
I liked Animal Behavior College at the time that I went through it, but it IS rather rudimentary if you have already been a professional trainer for a while.. It isn't all pure positive, and that is one thing I REALLY liked about it, because SO many schools ARE all positive, and I am not.. I like the fact that I went through and finished ABC, and I am very happy to have those initials after my name. It is also very affordable. my payment was like, $30 a month.
The very best thing you can do, in my opinion, if you want to be a dog trainer is to TAKE CLASSES, no matter how much you know - or think you know ;). I have been surprised many times, and it is research to take classes as well as continuing education...
I got certified through PETsMART as well.. That, if you ask me, is really a good way to get started because although it isn't a very thorough education, you will get what you put in. I trained for PETsMART for years and that is how I got into dog training in the first place... Training for a "big box" store like that gives you access to SO many different pieces of training equipment and SO many different dogs and people.. It also helps you hone your sales skills, because let's face it, if you can't sell yourself, it doesn't matter how good of a trainer you are because you won't have any clients if you can't "sell."
There are a few other good schools out there... Penn Foster (www.pennfoster.edu), CATCH Dog Trainers (http://www.catchdogtrainers.com/) <--- looks very cool, but is more expensive and I haven't gone through them yet, although I have got information from them, and then it gets more specialized and more expensive from there.
The BEST POSSIBLE STUFF that you can do if you want to get into dog training is to seriously get out there, get gritty and get your feet wet. Volunteer at shelters, take classes with your own dog(s) <--- g*d forbid, if you don't have one, borrow one!, watch youtube videos on training, read books, join clubs... GET OUT THERE and immerse yourself in it! There is NO substitute for experience!
Also, the usual way of individuals becoming dog trainers is to shadow one you can relate with. I STILL would shadow the trainers I respect, even today, with pride! :) So how long you do it all depends on you. I usually have an apprentice one way or the other :)
I hope this was helpful... Please let me know.. I am happy to answer any questions. Keep in touch!