Accounting system of some sort (Quickbooks), and then your biggest challenge will just be marketing and getting customers. 

@Kelly Rogers  When you file is ready in the system with your lender they can run the file in so many ways to give you advise which debts you can pay so you have right DTI for loan.

I will have to install a new septic system, and well for these spaces as well.

Your housing systems: heat, electric, water, structure should all be functioning correctly - check those.

What phone system software do you use to take inbound phone calls from sellers?

Obviously it is code to have the ability to escape both bedrooms in the event of fire, but could this be mitigated by a sprinkler system or is this effectively a 6 unit apartment building?

With better equipment as lights, HVAC systems?

Can anyone recommend a reliable HVAC company to service (check out system) and possibly do duct cleaning?

I am a full time employee and get my taxes at H&R Block.

It sounds like you've got a solid system sorted out.