Do I need to block open deliveries as well?
I can't advise for blocking both Sales Order and Delivery without addressing the scenario. Please let me know you scenario First?
You have mentioned that your client used the system 5 years back and also you want to use your system as fresh. If I am right then you should contact your FI/CO Consultant to close/reopen your Financial statements, Balance Sheets, Books, etc., for the fiscal year 2014 in the system.
As per final discussion with your Finance Team you can go ahead.
If I am wrong, can you explain your scenario in brief as I mentioned in my first line?
Anandkumar KS