What are the biggest pitfalls I should look out for? 

One of the biggest red flags I have as a lender (who lends my own money as well as secondary market loan) is when a rookie tells me there goal is to buy 10, 20, whatever properties in the next year. my next question is how many do you own currently?

It's the biggest unit of the 4 with the highest rent.What can I expect with this tenent?   

Biggest hurdle right now is finding a reasonable cleaner.

Ones of the biggest was that we had to completely replace all the old electric, luckily we were able to save in other areas of our budget to offset most of the costs but not all of them.

My biggest tip here is work with the smaller companies.

Depending on the lender and program your biggest hurdles might be credit score and income and time in job. 

The biggest factor to consider is the new interest rate since it’ll likely be higher than your current 3.2%.

This has been my biggest discovery, in life. 

The agent will have access to the biggest platform to get the property sold though, the MLS, so forgoing even a cut rate agent will create more of a loss than the agent fees themselves.