There is so much I want to say on this one. But I will be short and sweet because I know there is a bug in GP2013 with email out statements the way you are as PDF. However if you are getting this to work that is great.
When you are in the statements window and ready to process the statements you can Email options and add the subject line there. I don't beleive you can add any body to the email however.
If you want to go the other route and use the second option you described as graying out your send email statements field in the customer card you can do it that way to. However your customer's email addresses then need to go into a different area on the customer card. Looking at a customer card you see where the address goes and then the globe (world) icon; click there and that is where the email addresses need to go to use this type of way of sending statements. I have a script I can give you to copy those addresses from the one field to another. If you would like that you can email me at khosman@boyerassoc.com.
I know that probably is super confusing so email me if you have any other questions.
Kristen