Will a past history of drug or alcohol abuse count against me in an evaluation?
Well, it's unlikely to work for you, but it doesn't have to be fatal to the evaluation. How seriously your past drug or alcohol abuse is taken depends on several things:
How long ago was the substance abuse? If you stopped using drugs a year ago, great. If you stopped using drugs last week, don't expect the evaluator to impressed with your new-found sobriety.
What substances did you abuse? Marijuana use isn't considered to be particularly bad anymore. Stronger drugs like cocaine, heroin, and the like are a major cause for concern among evaluators, however.
Past convictions and/or jail time for substance abuse are also major 'red flags' to evaluators, as these things usually indicate a deeper involvement than just casual personal use.
If you have a history of drug and/or alcohol abuse, you'll need to show the evaluator that you've cleaned up your act, made sensible choices about the priorities in your life (namely, your children), and that you've not only ceased any substance abuse but are unlikely to return to it.