Ask and ye shall recieve.
Well having the Gaz is a good start. Now as to what you should pick up, there are number of choices, which mainly depend on two factors. 1) Do you forsee yourself running Scarred Lands? 2) What percentage of the money you spend on RPG products do you see yourself spending for SL prodcuts?
If you say yes to the first and only consider about 15-35 percent, then I suggest you get Relics and Rituals and then add Creature Collection 1. This way if you wants to RUN but only want mininal understanding of the Scarred Lands, this is the way to go.
If it's MORE than that, Then R&R AND add the the Divine and the Defeated. You might want to add the Wise and the Wicked too, but definately the first two. This way you get ALL the necessary components of what the Gaz offers AND add to some of it understandings.
I hope that was helpful.
BUT I DO more than recommend GET THE SL DM's SCREEN! It's ESSENTIAL for any good SL DM to have.