Advise on Chicago 3 Flat needing major renovation

Looking for advise. I have recently been tasked with getting a 100 year old brick three flat in Rogers Park Chicago rent ready. The building is fully paid off, and the owners, my parents, are retired and on a fixed income. Two of the three units have been vacant for almost 10 years (now they are all vacant) and there are years of deferred maintenance (original windows, lighting, wiring etc.). The long term goal is to do a gut rehab from the bottom up, but this will require a sizable amount of money (each unit is approximately 1800 sq/ft and 30 windows per unit) and the owners are not interested in taking money out of the building for this right now. They have approximately 30,000 they can afford to put into the building for an initial cosmetic fix and getting units rented.  The idea being to get the units rented quickly and as money is saved do full rehab of units. The rent money after costs will go right back into the building. I am looking for advise on how you would recommend eating this elephant. Also if there are any professionals that you would recommend bringing in that would be able to give insight as to how I should proceed and help develop a game plan. Thanks. Sorry for the long post.