Yes, ignore the ROI.Yes there is a formula that you use:All income (cash flow) at year's end - all expenses at 1st year's end = Cash on Cash Return        cash you used (cost & fees fro HELOC for 1st year) at start

Avoid those expenses like the plague.

Basically you assume several exceedingly bad and increasingly unlikely scenarios and see what your finances would look like.For instance, look at what your numbers would be if 2,3 or 4 units went vacant and you had large capex expenses like a new roof.

Closing Costs in DE are expensive, would I pay those twice?

The expense you incur is your "tuition"; and modify your process in order to never, ever make the same mistake twice.

I recommend snowballing when you have a 6 month emergency fund in place and can easy pay all of your monthly living expenses.   

In certain cities, it is very expensive for water service and some charge every month even if the usage is zero.

It's not too expensive to get started but it is expensive to maintain.

It's not that expensive.

The 50% rule is related to expenses, which does not include debt service, so your down payment won't affect that.If you're not planning to live in the property, you will have to put down at least 20-25%.