If I do change it out, would you recommend fiber-cement/hardi or LP or what else?

The other half of the house had carpet and linoleum we took out and has a cement slab underfloor.

It is hard to tell from the picture, but it looks like it is cement.

the lawn'Seems that they dug up every last bit of sod and stacked it on the cement pad that should be the parking area!

If you're a commercial investor, look for lots with chain link fences, cranes, cement trucks, job trailers, and contractor signs and talk to people -- sign in at the front desk, wear proper PPE, observe posted safety protocol if you enter the jobsite. 

Cement down drains, seen it.

I just build a 24x24 garage at a rental property of mine and built it by myself, outside hiring out the electrician and cement.

I spent more on PVC and PVC cement than the rest of it combined.

It's common to hear stories of cement poured into toilets and holes bashed into walls. 

Just remember if you write up doc to cement the partership, EXPLICITLY spell out how the partnership will break up, spending time now, when the deal is good, mapping out what will happen when the deal is bad, will save you much time and headache.