Although a 6-unit is smaller than the size of the CRE loans I have dealt with, some of the same credit analysis might apply.  

A house exactly the same size and rooms sold for 380,000 on the same street.

Again it depends on deal size and whether for example STNL or MTNL.MTNL ( strip centers ) when putting 25% down break even occupancy tends to be about 68%.

It occurred to me it may be best to build a garage/adu or large addition in order to capitalize on my lot size and the fact that I am zoned for 2 family.I am not sure my town will allow a detached garage with apartment above, but since I am zoned 2 family I should be able to add an addition with minimal/ no variances.

That size park can afford to hire part or full time people to deal with a lot of day to day issues.  

You may want to use your local GIS map for tax information which will typically give you current owner address (important for direct mail), bed and bath count, square footage, lot size, year built, last sale date, last sale amount, current year appraised value, etc.

Usually 15 years is not figured, People usually get 30 year loans, but depending on the size of the multi family most of the times they are commercial loans and are usually 20 years.

This beam could be a difficult item to replace due to size and access.

If the zoning has changed (setbacks, density, etc) then you will be able to put back the same size structure that currently sits there because it's grandfathered in.

The quality of your deal and the temperature of the marketplace will dictate the size of your fee.If you are double-closing, your buyer will pay a higher price than you are paying your seller.