If you're looking into a building that is already occupied (say 80%), you need to look at a couple of different aspects of the condition of the units considering rehab.
First, are the vacant suites rent ready? If not, what's the estimated rehab cost for those suites? How long will the rehab work take?
Second, do the occupied suites require rehab, or are the tenants happy with their current condition?
I have a 44 unit building that I'm currently renovating that is about 55% occupied now. I have been concentrating on getting the vacant units rehabbed and rented out as fast as financially possible to increase cash-flow. I have not done any rehab on occupied suites unless there were health and safety issues, or obvious issues like damaged drywall or leaky plumbing (water is your worst enemy!).
For tenants wanting major renovations I have given them the option of moving into one of the newly renovated suites, however, they will have to pay the higher rents that those suites now get. If they don't want to pay the higher rent, then I will do minor updates to their existing unit like new paint or carpet if it needs it.
This doesn't take into account other rehab that a building may need like roofs, boilers, windows, hot water tanks, common areas, or parking lots. There are so many items that can suck up your rehab funds quickly.
Good luck, and keep us updated on your progress!