As a contractor I always prefer a full gut but it does eat away the rehab cost and takes longer but no stress about plumbing, leaks, electric, lead paint and etc in the future.The patch up would cost less, fast timing, but then you are gambling with what's behind the wall and hoping nothing happens in the future.I assume most house that are purchase for BRRRR purpose are in distress/bad condition and old on some occasion.What rehab number would you go for?

After the inspection it comes out that despite the structure being sound, HVAC electrical plumbing and roof are pretty old and may need attention soon.

The oven has an electric igniter that has to remain on while the oven is on.

Now when we look at the surface items (flooring, kitchens, paint, lawn care, etc that is pretty easy to replace but that tenants habitually destroy) vs the items that really matter and can suck up a lot of funds (electrical, bathroom, landscaping, etc.).  

I have been researching to see how much it would cost to rehab the house but I am sure the roof, electrical and panels will need to be redone.

If you wanted to fight one mortgage for both units, look at the electric meter and water meter for the building.

3 apartments - probably a C+ to B- building in a C location:#1 2BR Rents for $600 currently#2 1BR Rents for $480#3 Efficiency rents for $500.Apartments 2 & 3 are on the same electrical, so their electric is included.

I certainly wouldn't suggest to pull an electrical permit to change an outlet or a plumbing permit to change some cast iron sewer out to pvc. 

Is the 4 plex individually metered water and electric? 

The house I'm buying has no finished floor or electrical fixtures; its a blank canvas.