Hello @Kelly Kormos One of the most important things to consider is the loan you will be using (or if you are paying cash). Depending on the financing you use the numbers will be significantly different. I am also curious if you are planning on keeping the current tenants or moving into one of the units to secure a FHA loan to house hack a little bit.
Although making sure you aren't spending more than market value for the property is important but making sure the numbers work out is more important. Bigger pockets has some great tools to help you calculate the cap rate for a duplex. You should definitely receive the rent roll from the seller. Other than that schedule a time to see the home. You will need to give significant notice because of the tenants but you will definitely want to check out the interior before putting in an offer. Or make your offer contingent on interior inspection. Also walk around the neighborhood a little bit and determine what kind of renters are likely to live there.