At this point I would like to sell it, and my realtor is saying that it’s marketable only to investors since owners/occupiers will not be able to get a conventional mortgage (and the insurance will be prohibitive).Is that true?

There are conventional mortgages out there for 3% down.

Otherwise, you could look into non-conventional loan options (outside of Fannie/Freddie) that allow for less than 20%, such as local credit unions.As for your numbers, you're missing a few important components; the big items would be maintenance, CapEx, and utilities (water/sewer/trash) for a 4-plex.

There are a few local lenders that offer 3% down conventional options.

In everyones experience which banks offer the lowest down payment on conventional loans for investment properties?

Does anybody else have a Costco Executive membership and get all conventional loans using their mortgage program?

If you are going to get a conventional loan or a fannie mae freddie mac loan or an fha loan you're gonna need to have that asset in your name because then they're looking at that home being a personal loan and so the banks are gonna require you to borrow personally.

Rates are still pretty low conventional, so you'd be leaving some gains on the table if you go hard money/seller financing. 

We go in at 530K best offer I’m going to do taking into account down payment, conventional Loan, payment, etc.

When you hit the limit of what FHA will allow then you switch to conventional at 5% down.