There is no one answer.....it depends on soil conditions, loads,etc.

Most of the world, the land under the surface, (and all of the oil $$, soil, minerals, water, etc.), are owned by the government, so you only actually own the property on top of the land, this is called the Feudal System.

I want to do a bit of work on each unit (add a half bath, finish the attic space, rip up the carpets to expose the hardwood underneath, etc.). 

Any chnace there are any soil contaminations?

We bought at or around market price since the house we were renting was bought underneath us.

We agreed on $40k for complete plans through city approval which include engineering work, but I pay for soils and topo map separately.Thoughts?

Today, you'll probably not hit much but once the correction comes (no one knows when it will, but rest 100% assured, that it will most definitely come) , all these people buying off projected numbers will be the first ones putting these properties on the market at hefty discount just to get out from underneath the liability. 

We routinely rehab houses built in the 70's and 80's and with the Houston soil [high clay content] foundation repair is common.  

Leave open space underneath for a trash can or counter stool.Are you replacing the floors? 

Flooded basements is such a common problem in some midwest markets that I did a whole podcast on the subject. 99% of water intrusion is due to improper drainage and soil grading and can be mitigated by having properly functioning gutters and downspouts and grading the soil away from the foundation.