Dreamworks are good, I've seen some of their work before. Price will depend on the damage and what type of paint it is (ie, metallic, gloss, etc). Without looking at it, it's hard to say. Could be anywhere from $200 to over a grand. From your description, it sounds like they will have to completely respray your side skirts. You're probably looking at about $300-$400 for that if they do the prep work. The damage near the fuel cap sounds more serious. To do a good job, they would have to completely respray the panel and then blend into the panels next to it. I'd say you're looking at upwards of $500 for that. But best to take it for a quote and see what they have to say.
As for whether it is the original paint or not, look up the paint code in the engine bay and then go here to confirm the colour: http://www.skylinesaustralia.com/forums/topic/138419-nissan-paint-codes/
If it's not a match then it isn't a big deal, paint shops have a device that they put on your paint to tell them what's in it so they can determine the colour. So it won't be a problem either way.