"If you want."
Here is a link that may help with your questions. http://www.guidetojapanese.org/learn/grammar/conditionals
If it is clear from the conversation before the sentence "If you want." that the person who wants is "you", then you could get rid of あなたが. Also, since もし is only there to stress the "if" meaning that is included in ければ, you could get rid of もし. The result would be 「ほしければ。」「ほしいなら。」「ほしかったら。」