Getting a PS3 for Ratchet & Clank Future. What about PSN

You've over complicated this.

For PS3, basically once a DLC was installed on a system, any user on that system can play that DLC and earn trophies for it, whether signed in to PSN or not, I've never seen DLC which is only usable by the buying user except for items like Home items, Avatars and DLC for one specific game I know of.

So your "process" is fairly simple:

1. Log into the appropriate store for your game's region (you'll need another PS3 profile with a PSN account in that region)
2. Buy DLC from that store (funds could be from a local credit card or prepaid PSN card) and install it.
3. Log back into your main profile and account and play the DLC.

PSVita is different. A PSVita system and memory card is locked to a single user, so even if you switched the Vita's user (system restore), you'd still be only able to download the DLC to a memory card assigned to the "foreign user", meaning it won't be accessible to your "main user" (which will need its own memory card), this means that in order to buy DLC for a PSVita game, you must own a copy of the game from the same region as your main PSN user. There's no current way to avoid this (at least one that I know of), and it basically means any PSVita game that has "required DLC" like online passes, UPlay pass, etc, will only be "fully playable" if you buy games from your own region.

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:

1. On the XMB (cross media bar), go to the Game section.
2. Go to Game Saves
3. Put in your memory stick (cf, ms duo, sd)
4. Find the game save you want to copy/backup, highlight it and hit the Triangle button.
5. Choose the menu option to Copy
6. 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!