All important factors to consider- population growth or decline, job growth or decline, inventory rate and movement- is is a seller's market getting stronger or softer or a buyer's market getting stronger or softer?

Having all Section 8 tenants may cause you to have a higher cap rate on your exit due to a higher headache factor surrounding Section 8.

Determine the most likely sales price of the build (factor in the cost of the land also) and include all selling costs to arrive at your net profit and ROI. 

The discipline to run on a budget for a number of years or decades depending on your math, doesn't factor in emergencies or market changes where the HELOC could be used to invest in other places.

I guess it's just too many factors to consider and it easy to get overwhelmed and cannot get accurate info and estimation when I conduct analysis (ex. property tax rate, insurance, maintenance fee, rent, repair, closing cost, etc.) 

Factor in carrying costs (utilities, taxes, insurance).2.

Does size of mortgage paydown become a factor here?

An LLC is potentially one minor factor that might contribute to the evidence that you are running your rental activity as a trade or business, but it's definitely not required that you have an LLC to qualify.

Those are some factors that will determine your returns.

That'd be hilarious.