If you are borrowing for renovations, you will likely have a draw fee in the $100-300 range per draw.Selling costs: biggest one is commissions.  

In addition, if i used fixed rate financing, my biggest expense is fixed.  

What are the biggest pitfalls I should look out for? 

Rent out to roommates so I employ the "rent by the room" essentially.Pros:insane cash flowif your room offer is much better value than other places to live, tenants will stay awhilea small community can be built when your tenants become friends with eachothernever ending supply of tenants (at least in my area)Cons:CAN BE management intensiveWILL BE management intensive until you gain experience and know how to manageIf you employ this strategy, the BIGGEST risk/liability mitigator is being VERY clear from the start about your expectations and that you will evict if new tenant doesn't follow your rules.With my setup - I furnish the rooms and have all utilities covered. 

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.