Congrats! That's an exciting step. And hiring someone you know so well already is brilliant. There shouldn't be any surprises!
Having never done this myself...I've seen NPs used successfully in various ways depending on your population and her strengths and weaknesses. I think your idea of having her do the first new patient is brilliant, it not only saves you time, it makes sure everyone knows her and is comfortable with her. Where I go, I can always see my MD if I want to but I can often see the NP at funny times like Friday afternoon or 8am in the morning. I can also get in with her immediately. My kids actually prefer her to the MD and I really do like her a lot though as an MD myself, I prefer to see the MD for most things.
I could also imagine a midlevel being super helpful with chronic disease management, making sure everyone is up to date on everything. I've known some practices where the midlevel sees DM/HTN/etc every 3 months and the doc annually. I personally would use a midlevel to make sure every single person in the practice is up to date on all the random screening (depression screening, hga1c for diabetes screening, etc).
If only there was a good way to compare with other offices in your area. Also, check with her about what's important. For some people, having health insurance is more important than a competitive salary and I know an RN who is a single mom and was willing to forego a pay raise in return for having a space at the office where her kids could stay after school so she wouldn't have to pay for a babysitter.
I know that @lisainden and her midlevel are like a right and left hand so I'm curious to see what she says.