What look for/look out for when purchasing a rehab house

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I'm hoping to get into rehabbing soon, and I just want to know from anyone with experience what are things to look for in a house that is worth rehabbing. Also, what are things to look out for that make the house not worth rehabbing.

Lastly, what is the maximum one should spend on rehab costs in order to still make a decent profit on the project? Is this determined by a general percentage of the house value, or does it vary depending on the project?

Thanks for the help!