My partners and I actually just mailed our LLC application last week so I just finished a LOT of research!
A lot of your questions will really depend on your individual state's laws. The best thing to do to get accurate answers is to contact your state's secretary of state office, llc is usually handled by the corporation division.
From what I've seen in my own research (we considered NV, SC, NC, OR, and decided on MS)...and as you said, I'm no lawyer and would appreciate any corrections...
Street address: should be a physical address. This address is the one used to serve papers upon an LLC in the case of legal issues (registered agent)
Purpose: Should be a brief overview of your intentions, what you plan to do with this endeavor. Sort of like your mission statement.
Operating Agreement: Depending on your state, you may not be required to have a written operating agreement. Each state has their own default laws regarding termination, adding partners, responsibilities, and operations. Most states allow the LLC operating agreement to over rule those default rulings - most states also say that the agreement doesn't need to be in writing.
In my opinion, you should read over your states laws regarding LLCs and write down anything you would prefer to handle in a different way. Your articles of organization filed with your state's SOS should suffice as evidence of your LLC status - I would think anyway :-)
Foreign LLC - This I'm not too sure on, and would really appreciate correction, but I think that since you will be performing the work in your home, it is considered a business in your state.
We are 4 partners and live up and down the east coast, including Canada. When I explained this to my state's SOS office, I was instructed to file as a domestic LLC.
As for the usefulness of becoming an LLC - it does add a professional look to your business name, but your work quality will be the determining factor of your success.
If your industry is known for copyright troubles or lawsuits, LLC can protect your personal assets in the case of a lawsuit. I'm not sure of other benefits for a sole propreitor? Maybe someone else can provide more info? :-)
Hope that helps some and you may want to call the SOS to doublecheck!