Plus so many companies moving and developing here like Amazon, FB, Google, Nationwide, Honda, Ohio State University, Intel, etc.

Follow economic trends and determine which areas have the best opportunity for economic development and prosperity.At the end of the day, you're looking for investments with the highest returns. 

I am considering working with a property developer in NJ to buy land, build a duplex/triplex and sell it (6-9 month time horizon).  

Investment Info:Small multi-family (2-4 units) other investment. 17 luxury duplex development located off of Galm Road in the vibrant growing North West San Antonio.

I've also developed a spreadsheet for a TRUE rental income calculator—which goes beyond just calculating net income—by incorporating many of the parameters you mentioned.Do you think it’s a good idea to buy a new construction house from builders for the first deal, especially considering aspects like tax offsets and depreciation?

Once you've found your target market, start developing your teams (Investor focused agent, Investor friendly lender, Property manager & contractor).

Quote from @Daniel Huang: Hi, I am trying to transition to develop my rental income as I am in the final years of my professional career.  

Once you start hiring employees, it's smart to develop it out so you can train them properly and so there is a documented reference guide.

Conversely, a new built A class SFR, in A class market of significant growth via a well laid out planned city development, great schools, area people are glamouring to move to, a-class tenants, very very low risk.

You can see the development in the area is in full swing.