Rehabbing house with known mold issues

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How much of a concern should mold be when buying a property to rehab? What does cost look like? The property I am looking at has 4 spots that will need to be addressed, two are on an outside wall, one potentially in the kitchen ceiling and then in the unfinished basement. How hard will resale be should I choose to sell versus rent? I assume I will need to have a professional come in to do the removal due to liability risks if I rent it out. What does the cost of a professional typically range? I tried to google search this but it is difficult to determine since the ranges vary based on severity and I don't know what one would determine a mild versus moderate issue.