HOA, shopping center,  or neighborhood restrictions?

Area, tenant use, tenant credit (i.e. only Moody's BBB or better guarantor credit).There are a lot that can vary in NNN, just like any other real estate.You have market risks: FL has another hurricane coming, CA has earthquakes and wild fires, the midwest has anemic population growth.You have some location risk: is building on hard corner with signalized access or buried mid-block with no turn lane?  

As much as I hate annuities, you might speak with an annuity agent about the employer awarding a bonus in the form of a single premium deferred annuity invested in index funds and that you do not have the right to access until a date certain and then a full payout over time.

Also, consider the potential for appreciation and rental restrictions, as some HOAs have rules that might limit leasing later on.Given your $20k starting point, a condo could work well as your first step into real estate, especially if you focus on one with a manageable HOA fee and potential for appreciation.

I decided to open a HELOC on my primary residence, this allowed me to have access to capital if I came across an available deal.

We do not do Syndications; we have access to all the funding we will ever need, and I don't like sharing the profits. 

I'm biased and can provide reasons and data to show that using an agent will protect you, get you more money, less stress and so on.The reality is you can do it yourself, but tenants generally don't cooperate with showings and that causes lots of problems for buyer to access the unit to show, inspections, appraisals etc.

The property has about a quarter mile of street access, and no trees or anything to block it, so I wanted to put something there to give a little privacy anyway, and then these trees obviously have to go, so I was hoping to re-use them.

Normally, unless there are specific building code restrictions, the amount of units allowed on a lot is determined by the Floor Area Ratio. 

Using a Self-Directed IRA (SDIRA) allows pre-tax dollars to grow tax-deferred in real estate, but limits flexibility due to IRS rules and potential UBIT on leveraged properties.Investing through a Roth IRA offers tax-free growth and no RMDs, but funding limits and restrictions apply.