I'm up to 10 doors (including personal residence) and have six more under contract.

As that will at least help folks that you reach out to know that you are a real person, with a picture and professional bio.

Personally, it's not for me. 

Here is the current list of cities:http://ritaohio.com/MunicipalitiesI am rather sure that the list has only been expanding over time.As a person/entity who earns rental income in any of these jurisdictions, you are required to file a return not only for the state, but also for RITA.

Our goal is to sell the first plat with the house and use the money to build a modest modular duplex on the other plat to rent.We are both very inexperienced when it comes to building and renting, and need all of the information we can get- what are some unforeseen costs associated with building a modular?

As others have mentioned, there is a personal aspect to doing it this way since it involves yourself, spouse, and possibly a family. 

If the IRA fails to pay property taxes or insurance bills - you are then personally responsible, which would be a violation of the IRS rules. 

Our personal  house had previous work done but the piers were poorly done, and we noticed dry wall cracks and tile floor "separation"  about 2 years back.   

Here is why there are no responses to your question: it's tricky.Prior to the 2018 tax reform, there was a deduction for personal moving expenses.

You will need to allocate the expenses between the personal and rental portions of the house.