Trust me Gar, you'll not need much for the Scarred Lands, especially if you run it as is. Better yet just let them get the Player's Guides, read them and let them be the judges. If they don't like it, then move to something else. (And send me the unused copies! )
I've been reading the Lands of Molokai pdf and would suggest it. It's cheap and big, with some good ideas. One thing that strikes me is how great the setting would be for Arcana Unearthed. It seems they had a premonition of Monte's Diamond Throne and I think you could easily plug the AU races into Molokai.
I haven't bothered to get a copy of Arcana unearthed, after reading the designer's notes a while back I was....heavilly unimpressed. So please take this next question as the honest inquiry it is: What is the "diamond throne"?
The Diamond Throne is the default setting for Arcana Unearthed. I don't know much about it beyond what is in the book, the actual Diamond Throne info will be in another product released later this month.