We have contracting backgrounds and I have a business/accounting/fraud examination degree.The sweetest part is that it's only 7 miles from my house and it's just the size I was looking for. :)

I would assess it's value based on a similar size/similar rent as a duplex or other rental in the area.

You kind of have to chart your own course.

Size of area? 

You're just buying a house to live in.A low ball doesn't make any sense.Comps are similar style, size, and condition.

depending on the land size it may be worth looking into setting up a wetlands mitigation bank.

I am contracting one right around the corner $300K half the size.

Legal use is determined only by the building department, which usually means actually going down to the building department, giving them the plat and lot, and seeing what they come back with as the legal use.I've encountered many times in my investing career (including just last month) where the tax assessor and building department had different # of units, and the building department is the one that matters.Usually the issue with making something a 3rd legal unit is whether the lot size is big enough, especially whether there is enough parking.

It's figured proportionally based on square footage so if the two sides are equal size then each gets 50% of the cost if not it's proportional.You'll have to get with a good Qualified Intermediary like @Dave Foster or @Bill Exeter and they'll help you get this all squared away. 

Call your local commercial brokers and agents that deal in that size space for multifamily.