personnel files and I-9 forms

I am developing a service that will allow small employers to outsource their day to day HR activities to me. Are there any issues with me managing their personnel files and I-9 documents at my facility (off their campus)? Obviously, I can manage the content better if they are in my control.
Comments
I also question how convenient it will be for regulatory agencies to audit files if they are off site. Something else to think about.
Good luck??