I am interested in a Tri-plex with all three units currently rented at about $1600 per month total( A little over $500 per unit, per month).

My question is about estimating total settlement costs as the buyer. 

Obviously you did not read my entire post or you would have seen the part about me trying multiple times to get it sprayed before baby came home. 

If you know the market well enough, then lack of disclosure is not totally a reason to abandon discussions though you should have some warning flags up. 

6 units in total.

It's obvious this is something that will be an inconvenience and you'll write your leases in a way that protects you from having a tenant trash the place to make it look like you're not maintaining the property. 

Numbers = $280k purchase, 25% down (required for MF investment properties), total mortgage all-in with Taxes = $1,500/month.

What i didn't piece together at the time and is obvious now.... there was a tree in the front yard.

Question: If FHA is 3.5% down, upfront MIP is 1.75% for a total of 5.25%, why would anyone ever take this over a conventional 5%?

Very little, we own 45 total houses that we rent, 2 4-plexes, a 3, 5, and 6 mobile home park, and the rest single family.