I figure around $2500-3500 depending on the number of breakers, the issue is that once you pull the permits you need to bring everything up to code. 

The strange thing was, every time I left Los Angeles and traveled a little distance a water heater in one or another building would go on the blink on a weekend and every tenant would scream about not having hot water and it is difficult to find a contractor who can repair most of these new water heaters with all sorts of new electronic circuit boards and gadgets on them.

So you have to ask yourself, are those issues deal breakers or would you be comfortable continuing forward, knowing you will have to tackle them after closing?

You will need to have (or add) - 2 or 3 - 20 Amp circuits to each unit to handle the new load.

That means seven bazzilionty circuits in the kitchen, one outlet on every wall in the bedrooms that is at least 3' long and one every 6' on each wall, tamper-resistant outlets where needed, GCFI where needed, etc. 

Circuit Court of Appeals and Supreme Court." 

In my area I go to the 13th circuit Court website and I can look up law suits and even speeding tickets.You should have something similar and I bet with a few tries on google you can find it.

For marketing I know myself I always try to lead by confirming the deal breaker on a unit.

Electrical- running basic circuits, 3 way circuits, lighting fixtures (fans, recessed, etc), basics on breaker boxes11.

I have a duplex (built around 1940) that has an older fuse style breaker box.