My parents own a 6 unit building (3 floors). The building is over 120 yrs old. It needs serious updates. I'm thinking a total gut (i.e. electrical, plumbing, tuckpointing, etc). They cannot afford and do not have the energy to complete such an endeavor. I would like to help them so that they can have a nice place to live in comfort as well as my disabled brother who lives with them. I have never done anything like this. Is it possible to renovate one unit at a time? 

Yes!

Start with the vacant units.

Get some income coming in.

Then take care of rest, one by one.

Good luck!

Thank you Max T. Appreciate the reply. 

So plumbing and electrical wiring can be addressed one unit at a time? I kinda figured that since the units are "connected" that doing work on one unit would affect the unit above or below...

Thank you in advance for your time.