@Anthony Bright This won't be easy. The bad credit is what will hurt you most. I would look to mortgage specific companies. See if any offer a Line of Credit. They almost certainly won't allow you to take 80% of the appraised value. Do you have a good debt to income ratio? Basically do you have more income from w-2 work coming in than outstanding debt and living expenses? My guess is if you go in asking for 50% you will stand a much better chance. LOCs normally come with a lower interest rate but it may be abnormally high because of the credit risk they would be taking. The good thing about a LOC is that you only pay the interest on what you have pulled out. So don't draw it until you are ready to purchase another property or whatever plan you have for the money.
Hope this helps a bit. I think it will be difficult but should be doable with a little research and time spent speaking with a lot of banks and mortgage companies.
@Anthony Bright , look into a HELOC. Very low cost in terms of fees. APR is higher than a mortgage, but you get a lot of flexibility. Just as importantly, what are doing to improve your credit? You'll want to get that sorted so you can get financing on any future REI.