However foundation and roof are your major items to look at. furnace you want to be 15 yrs or newer, I just replace and have new now, less hassles.

Some of it is quite specific to the area the writer invests in, but a lot of the ideas are applicable to lower class properties, and get you thinking about things differently (at least for me).We have started pulling out carpet and replacing with laminate or vinyl planks.  

At that point your cost to replace the term may be prohibitive.

We want to primarily buy and hold for passive cash flow in order to replace my monthly income within the next 18 months.

Hire an engineer to help you draft specifications and a create a solid contract that dictates pay apps.  

Roof, HVAC, and plumbing replaced approximately 8 years ago.

These are expensive to replace if you don't have to.   

The county or city may also want an engineer to verify,  and thats just the fee to tap.

I've been working in the engineering field for many years. 

English litt, might help you write compelling marketing materials to find good properties; Anthropology may allow you to understand different cultural shifts in the housing markets; history could teach a lot of market timings in Real Estate.I studies Electrical Engineering and Economics, neither of which gives me advantages in Real Estate.