The first apartments I rented as the owner, were previously rented by the former owner at $150 month including utilities. 

Cost segregation utilizes the MACRS approach.

As noted above, up to 50% of gross rental income normally gets eaten up by expenses (taxes, management fees, utilities, landscaping, repairs, capex, etc).

There are actually THREE kinds of subject to transactions1- subject to existing mortgage without lender approval2- subject to existing mortgage WITH lender approval3- subject to existing mortgage as part of a seller financed wrap around mortgage.with #1 and #3 it’s in everyone’s interest to utilize a third party servicer, who collects and then distributes payments.  

I talked to a modular home company in my area, and he said to build a 4 / 2.5 2200 square foot home would be 215k, and then foundation and utilities would be another 160k.

I’ve heard that this neighborhood doesn’t really have any problems from a social worker but outside areas aren’t good. 3/4 units are currently rented at $900 each, renters pay utilities.

@Alecia Loveless is correct--tenants need access 24/7 to their electric service panel, water and heat, assuming those are individual utilities for each unit. 

To minimize capital gains tax on the sale of their primary residence, your in-laws can utilize the $500,000 capital gains exclusion, potentially explore an installment sale to spread tax liability over several years, or consider converting the property into a rental to use a 1031 exchange for tax deferral eventually.

Another option would be to utilize a property manager to manage everything and keep a buffer between you and the tenants.

I underestimated utilities in my early years, so I ensure those are properly captured (electricity, gas, water and trash).