G
It seems like it is not so much a question of whether there is a requirement in 21CFR for you to comply to this, but more that you have a customer requirement for it. So, you may want to bite the bullet 
- No validation without proper training, you need a "project lead" who has a proven training within this area.
- Be happy, that the system is OTS, that way you can get around the task much easier - see GAMP5 for the requirements for OTS software.
- Apply the principles in Part11, and if you can't apply all of them then argue your case in the validation papers as to why you don't need all of it. FDA should accept the "least burdensome approach" and it may be overly burdensome to apply every aspect of part11.
- Keep the system under change control to prevent revalidation that is not necessary. Change control ensures that you can argue your validation status.
