Thanks everyone for all the replies. You've given me a lot to think about.
I really like the idea of the plane ticket to NYC, a night or 3 in the hotel and a stroll to
B&H where she gets to pick her own toys out. Unfortunately this option won't be in this years budget.
I'm thinking the D600 is a better long term choice, even though it's more expensive. I ran a bunch of comparison charts and the 600 even beats out some of the higher end cameras that are hundreds of dollars more. So when you consider bang for you buck, it looks like a clear winner.
I'm leaning toward the cash or gift card idea and a shopping day on boxing day. But I can't see the D600 going on sale too much as it's fairly new. So maybe money towards necessary equipment like speed lights, diffuser etc. One thing I didn't know is that the D3100 lenses she has won't be compatible with the new D600 if I go that route. That is real unfortunate and thanks to TheLost for pointing that out.
KmH, she mostly does shoots for family members and close friends at this time but is growing all the time. I have got the license but didn't think about some of the other stuff you mentioned like business liability, indemnity, or E&O insurance etc. Thanks, I will be google-ing what is required tonight.
Thanks again everyone for all input. I'm sure I will have more questions I will add them under this thread as they come up. I appreciate it.
Cheers,
Clueless