I'd been a member of audible for years, and only stopped when I moved and my commute was done by car instead of train.
All books I have are unabridged, and my wishlist was filled with even more of those, so, I wouldn't worry too much about that.
About the DRM, you can activate multiple devices, right now my laptop, my pc and my MP3 player are activated. You won't get MP3 files, they have their own format, which is much better compressed than MP3. But it would mean that if Audible (or rather Amazon) would pull the plug, you're kind of screwed. But personally, I don't worry about that too much. If you're really worried, you can burn them on CD as AudioCD.
And you can download them all on your PC and from there put them on your devices (using the audible program). That's how I put my current selection on my MP3 player.