Just as a suggestion blackemerand1, maybe you could get the dog of your choice, and then find a dog behaviourist to assist you with some training. This would alleviate any waiting time you may find with a trained assistance dog. Provided you don't require the legal status that Eleanor has wisely mentioned :-)
I know my dog would never let someone come to the door unannounced, and has at times, 'fetched me' from the garden when I haven't heard the phone ring. I don't imagine the alert training would be too difficult to teach, especially with some good advice and pointers.
That's so true, you get back 10 fold, whatever you put in, and the bond you form is priceless.
In my case I'd never leave the house if I didn't need to take the dog for his daily walk. At times that's very difficult for me, especially as he attracts attention I could do without, but I see that as my duty, and his right.
The up side is that I have some daily resistance to agoraphobia, and we know a bunch of people I'd otherwise never come in contact with.