You'll have no problem getting it to run smoothly. It's a very well coded game. I play it on 2 vastly spec'ed PCs and it runs great on both. On my desktop I have a quad core, 4GB, GTX460 @ 1280x1024 and it is silky smooth with everythin on high. On my laptop - Dual core, 4GB, 9800GT @ 1440x900 and it's smooth with it set to medium/high. You won't have any problem running on high at that res.
As for modding, this game is very easy to mod. I've done the black market mod (opens up all items), superman mod (changes you to be a flier without a chute), costume changer (let's you wear the outfit of any NPC, Ninja's my favorite) and massive explosions mod (makes you C4 like a mini nuke, very fun)
My copy is from steam and it doesn't mind. If you go to drop the files in the folders, you just have to go to Steam>Steam apps>common>Just cause 2 and most of the stuff goes into a folder called "dropzone" Very easy
Yes, I believe so. It seems to be a repository of sorts for custom modules. I think it was already there on mine. One of the readme files said if it's not there to create it, but I believe mine was there. Either way, the game looks for it. Very few mods ask for files to be dropped elsewhere, like a graphics file. I think the only one that did was the space rocket mod. I ended up turning it off because every rocket in the game creates a nuclear explosion which kills you and everyone. It was fun for a while but kept me from accomplishing anything.
I'm not sure. I guess it's for people that don't mod. Or it could be that it adds new items that weren't in the original game. I haven't seen it yet but I'll look. It would be nice if there were some new rifles and smg's. I get tired of the M4,SAW and MP5 being the only automatic weapons. That's my only gripe. Of course I haven't dug around on the mod sites to see if anyone's created any new ones.