It's possible to share PSN games between PS3 systems. There's a thread on the SomethingAwful forums that goes into a good bit of detail as to how to do this.
It appears to be at least acknowledged and ignored by Sony, but keep in mind that you should be very careful not to violate any of the terms of your PSN agreement, lest you lose your games or access to the online support of your console. This process is also not without risk, and your "friend" might do something dishonest and cheat you.
Do note that recently Sony changed the rules, so that you can only have 2 PS3 consoles sharing a single copy of a PSN game.
One: You can back up save files and then restore them at a later time even to another account. If you are a PS+ member you can back them up to a server and they will be linked to your PSN. However in your case, you'll probably want to back them up manually, which can be done as so:
- On the XMB (cross media bar), go to the Game section.
- Go to Game Saves
- Put in your memory stick (cf, ms duo, sd)
- Find the game save you want to copy/backup, highlight it and hit the Triangle button.
- Choose the menu option to Copy
- Hit the X button on the memory stick and let it copy.
Source: PS3 : How to backup PS3 game save files to memory stick / sd / cf
It should be noted that some games use copy-prevention methods to, well, prevent copying. However, it's still possible to back even those games up. Google is your friend here.
Two: I'm surprised the previous owner's PSN is still on there, but yes you could use his DLC, as long as his PSN remains registered on the console (so don't delete his User account.) Off the top of my head I don't know how this will affect trophies. You should still be able to earn DLC trophies, but they might register for the owner of the original purchase. (The PS Vita has this problem.)
Three: The PS3 is not region locked for games, so you should be able to play everything fine. However, I do believe the Playstation Store is region based, and so some content will only be available depending on which region's store you access. I'm not sure if this is determined by the console (e.g. a US PS3 vs. a JP PS3) or registered PSN region. Also, you may have some conflict if trying to purchase content that is normally not available in your region.
I hope this helps!
P.S. Congrats on the new PS3!
Best Answer
You can't log on to the same PSN account on two PS3 machines at the same time.
You can have the account present on both PS3 machines, but as you've observed, once you (try to) log on the second PS3 you get the error message, and you'll have to log off the first PS3 before you can log on to the same (with the same account).
As such, you will not be able to be logged in on two different PS3 machines with the same account.