Starting to look at a multi unit to "house hack"

[Adam Spencer]

Ok so I am starting to look at 2-4 unit properties to be a live in landlord. I will be using an FHA for the purchase. I have zero experience. I have a few questions regarding this plan.

First off as I'm learning to do the numbers using the buy and hold calc on here. I've found a few that look good but am not sure if I should use all rents I will recive fully occupied? Or should I put the gross rent as the amount I'll get from the other units? I'll also be renting out any extra rooms in my unit as well. 

Another question I have is if I'm looking at a 100% occupied multi how can I live in one unit? Can I ask the owner if any leases are up soon? What strategies do you guys recommend? What pitfalls should I look out for with tenants that are in place? 

Thanks in advance for the help guys