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The home is old, built in 1919 with original plumbing, wiring and two major floor to ceiling cracks in the foundation.

Much of the foundation is laid for you.  

I don't have any experience with basement leaks and/or foundation issues, so I probably would have avoided the place.Speaking of which, I need to learn more about those problems and the costs associated with repairing them. 

It's like marriage -- courtship takes time and is foundation building.  

If it was damaged by water, I am thinking maybe you need to make sure at least there's no foundation structure issues with the house, and also check if there's health issues.

I need to do a little maintenance on the roof and foundation, but it will be funded out of the repairs budget that was paid for by the rental income.  

You just need a good foundation so you don't make any critical mistakes.  

-The foundation has deteriorated at the front right corner of the house, the parging has come loose along the right side of the house, the foundation walls are bowed in on both sides of the house, and the mortar has deteriorated between the blocks and bricks of the foundation in several areas.

My question is, would a foundation company have to repair the entire building (i.e. all four units) or would it be possible to just pier the one unit?