Need Help In Purchasing Phase one/ Mamiya DF+ Lens and Digital Back

I have been shooting Medium format film and Digital for 50 years. I always wanted to try this system out.

 The purchase would be for my personal use. 

I only shoot Portraits and Family group shots ( Mostly Pro Bono) 

To keep within my Budget I am looking at the DF+ Body, SK 55mm Ls Lens and a Digital back that will allow me to shoot a Max ISO of 400 without NOISE.  I have been researching a few backs that might meet my needs: Credo 50,   Credo 40 or 60( not to sure about these)

IQ 180 back. Since I am New to Phase one maybe you can offer other suggestions. Thank you kindly.  Dennis