Are these common now?

Looking at pure spreadsheet math and common convention; if you pay down your mortgage you are effectively making that interest rate on your money. 

It was common back in the day.

The market has definitely gotten more competitive, and it’s common to see BRRRR deals over $250K, especially given the high demand/appreciation in the area.

Common fees will include a set-up fee, a leasing fee for each turnover or a lease renewal fee, marking up maintenance, retaining late fees, and more.

Just simple common sense. 

It is common for PMs to stick fees into paragraphs throughout the agreement, this is another red flag.

2-4 units are not that common, but builders are building them more now.

You asked about buying older properties and the biggest issues I have found is that roofs need to be replaced, HVAC needs servicing/replaced, and sometimes there are foundational issues (not super common). 

Commonly known as xxxx.