If you want help with it, ask in the cookbook thread
http://www.bay12forums.com/smf/index.php?topic=175538.0.
Setting the scarcity to a high value (I use 80000, a bit above Very Poor) results in many embarks with little metal. However, alloys (steel, bronze, etc.) are decoupled from their component metals, and so are available to compatible civs regardless. Thus, dorfs always have both steel and bronze for their caravans even if they don't have either iron or copper.
This means that you're likely to be able to "harvest" goblinite even if there is no metal on your embark. I assume you could change the civ raws to make alloys unavailable, but that also means there's a significant risk you can't get hold of an all important anvil (although if you don't want to work metal anyway, it's no loss, but you don't get copper weapons either, in that case).
If iron is very rare, you might still get hold of an anvil through aggression induced trade with humies and other dwarven civs, but it would probably be a challenge.
There are two routes towards your desert world:
1. PSV specification, i.e. defining the parameters for each world tile in detail, so the world is custom designed. Using that template, you can then generate worlds until you get civs roughly where you want them.
2. Setting the various advanced world gen meshes. For desert, you'd set Rainfall to be very low on average, but would have to allow it to vary sufficient to allow for forests (to allow for elves) in some isolated pockets. I don't know the details, as I have very little experience with that method (I use the first one).