M.,
I am going to be as honest as I can with you. I HATE the fact that so many parents BASH nannies. I am a nanny, with my own children, and I have been one for over 10 years now. My job as a nanny was a CHOSEN profession, NOT one that I am doing just till my dream job shows up! Sometimes there are people out there that do nanny thru college years while they are studying for a degree, I originally was in school for nursing, and decided that I LOVED providing one-on-one care much more to families. I love taking field trips to the zoo, parks, going for walks, having picnic lunches in the park, and curling up on the couch with a movie, when someone's sick. I LOVE MY JOB!!! I thank God everyday for my job with parents whom LOVE their jobs! They aren't any less of a parent because they choose to work. Mom is putting away money for her daughters future, and loves working closely with me to make sure her child on the right track developmentally.
Daycares are awesome as well, if you find one that you love. My best friend worked in a daycare for 9 years before one of the families asked her if she would be interested in being a nanny for them FT.
You say your son has medical needs. While most daycares can try to be accomodating, a nanny probably would be a better fit in my opinion for your situation. Find a wonderful nanny, whom you can train. Remember that just because she may not have the college degree in child development or education, doesn't mean she isn't a good nanny. I have a medical background and have gotten plenty of offers for jobs based on my babysitting/child experience.
Good luck!