A lot depends on how passive or active you want to be and your risk vs reward tolerance is. 

Some of the best advice I think you can hear is : first decide on your risk tolerance, decide what is an acceptable return for you or your investors, then develops a simple model/ spreadsheet that indicates your assumptions, then plug in “as close to” what facts you can independently verify, then the anticipated unknown.

HELOC or Cash...if neither of these work than a business Line of Credit...if that doesn't work than a personal line of credit (assuming risk tolerance and adequate credit score)

US citizens won't tolerate it  They already proved they aren't the most compliant people on the face of planet Earth, even when "the science says." 

Most areas are homeowner friendly (or at least tolerant) and will help you through the process.

In order to use your retirement fund for investing in Real estate, you need to: 1) Have an understanding of your risk tolerance: Knowing your risk tolerance is important when using retirement funds because it can determine how aggressive of an investment you want to make with your money. 2) Be mindful about taxes: You may be subject to income tax on any gains from investments, which could lead to a reduced return on your investment and/or taxes due at year end (if applicable). 3) Understand the withdrawal rules: If you withdraw money before age 59 ½ without penalties, the IRS requires that it be included as part of gross income.

It will be up to you to decide which fits your situation, risk-tolerance & market best. 

That said, I’m willing to put up with a little more degree of difficulty for an automatic check.Depends on your goals, risk tolerance, investing area, etc.

I have a flexible schedule, am a very fast learner, have a good amount of capital to invest, excellent credit, a high risk tolerance, and am willing to get my hands dirty. 

It depends on your goals, your market, and your risk tolerance.