I'm in the process right now. I can't knowledgeably answer your first or second question, but I can address the third from my own experience of doing the paperwork myself...
If your situation is very straight forward (no dependents, no criminal history, no serious medical complications, no previous marriages) and you are comfortable filling out forms, it's worth considering doing it yourself.
http://www.cic.gc.ca/english/immigrate/sponsor/spouse-apply-how.asp Is the place to start. They have guides and document checklists and everything.
However if you are uncomfortable with forms or your situation isn't as straightforward as it could be; an immigration lawyer will make the process a lot less stressful. Having an immigration lawyer help you with your paperwork will not make the PROCESSING time faster (although it may make the information gathering stage faster because they know exactly what to do without having to think it through.)
Having done the paperwork myself, I can safely say the wait can be very daunting and self doubt begins to creep in...Did I forget to sign something? Did I include everything? An immigration lawyer gets a lot of money to make sure that it's complete.
If you trust yourself to handle all the forms, go for it. If you'd rather spend the money to have someone else make sure that all the t's are crossed and i's are dotted there are literally hundreds of immigration lawyers out there. It's really personal choice.