Are you doing newspapers?
Overdrive has a pretty limited amount of back issues for magazines, which is why I generally ignore it, except if I want to quickly read something on my Kindle.
Do you have a plan for all the data once you have it?
I think your estimate of magazine issues is high, but I commend your effort. I wouldn't mind hearing about which you've managed to complete. I keep finding that I can get all the historical issues, and I can get all the recent issues, but there's this hole in the middle from about 1985 through 2005 that is usually impossible. It's irritating.
I might test if I can pull audio books down too, though those will be limited to the ones appearing in my own local library (every municipal library seems to have access to the full magazines though, if you know the magic urls).