- A single story is less maintenance cost while a two story house can divide the area into private space and common space.2) Which is more important for students, private space or common area (bedroom vs living & dining)?

Shared expenses such as insurance or roof repairs are usually divided based on square footage or number of units, while costs that only apply to the rental are fully deductible.

Buy 80 acres and divide into 2 to 8 acre lots.  

The totality of my plan is to divide 3 equal lots, build economical but marketable homes on each, rent/sell them, then use the equity from them to collateral our forever home on the remainder of the property.

It is entirely dependent on the variables above for each and every monthThe amount of interest paid in each payment is: the interest rate of the loan multiplied by the outstanding balance at the beginning of the period, and then divided by 12.

Should I just sub-divide the land and build a duplex on the land and move in one side, rent out the other, and then rent out my single family home?

Try taking the replacement cost of every major system and dividing it by the remaining lifetime of every item.  

Smaller spaces are still in demand , if you have one big space and can divide up somehow cheaply then that can work but remodling does come with price espcially in office space. 

Figure out your Net Operating Income and divide by the local market cap rate.

The totality of my plan is to divide 3 equal lots, build economical but marketable homes on each, rent/sell them, then use the equity from them to collateral our forever home on the remainder of the property.