buckworks - 8:30 pm on Nov 4, 2013 (gmt 0)
This sounds like a job for a custom combination list.
Create a list (or lists) based on users who have reached your subscription confirmation page, logged in, or visited any page on your site that would only be reached by people who are already customers.
Then create remarketing campaigns that are targeted however you want, but EXCLUDE the above lists from those campaigns.
You can mix and match combination lists any way you want, including setting different cookie lengths, to include or exclude different people from different campaigns.
One group that is often worth targeting separately would be those who started out to subscribe or purchase but didn't complete the process.
You don't actually need a lot of PPC history to start remarketing, you just need to think carefully about how you're going to segment your campaigns going forward.