I am currently in the process of rehabbing a former slum duplex. The previous owner got it from auction with the intent of house hacking it, he owned it for five years and never got past the lower livingroom. He allowd the roof to leak for years and just shut off the rooms that had water damage, both upstairs and downstairs.
I got the property for what he owed in unpaid taxes and a lean on the house. My mistake was self financing the rehab, and trying to do all the work myself, after my day job.
Get good estimates from good local contractors. Assume any least one large miss when estimating.
If you have a good plan, and your numbers are good, don't be afraid of hard money lenders for the construction funds.
I have seen two approaches,
1) rehab to a better level, everything works, and fill the units to get cash flow. Plan larger rehabs later as units turn to get the rents higher.
2) do the bigger rehab right away to maximize rents in your neighborhood.
Get your units rented and refinance out to pay off the hard money lender.
then repeat.