Consider tankless heaters for each unit so that they can use the solar array as a preheater but also heat as needed.

Depending on the location and orientation of you roof I would look at solar water preheat there are tax credits and financing that makes the payback period pretty short.

Solar hot water systems (popular in the Carter administration), used large, usually tilted up panels on the roof to preheat hot water or glycol in order to lower the homes electricity and/or gas consumption and to heat hot water.

I caught something on fire in her kitchen (not my fault by the way someone had left plastic cookware in the oven and i assumed it was empty and turned on the preheat and went to my room to fold clothes).

Even in our northerly (read long, cold winters) climate, a modest (8-10K installed) solar thermal system met 60% of the DHW needs of a 6-unit building and meant the boiler did not need to fire at all during the summer to heat hot water.Such a system can also be used to pre-heat the return lines of a hydronic baseboard/radiator heating system lessing the load on the boiler.  

Alternatively, you can install and HRV in each unit which will cost more, but will provide better fresh air distribution and stale air removal while pre-heating/cooling the make-up air being brought into the unit.    

The original can have it's temperature turned way down and act as a pre-heater to the new tank.

to preheat the water going in to the 75 gal. 

In most situations, running an electrical circuit is easily accomplished with minimal damage and rework.120v in-line heaters are meant to be used as boosters for water that has already been pre-heated but where additional rise in temperature is required - the most common examples would be in washing machines which have steam clean or sanitary cycles and in dishwashers.  

So you need to get clear on your processes and definitions, you don't pre-heat the oven to 25 bananas then filter the blinker fluid into the chakra through the hopscotch, that's just word salad.Your choice of wording, lack of clarity and completeness made my antenna go up, that's why I seemed to come back curtly.