I have thought about using a converted container that has been adapted for use as a rental space or, possibly a trailer, etc, or even new construction, but I have no idea what kind of cost I would be looking at. 

Convert to over under duplex?

Lastly, you have to get permits from the city to convert an SFR to a duplex.

Whether the loan is renewable or convert to perm. will be in the covenants.If your piece of land to build on has such an amazing location from anything else on the market then it might make sense to do it.    

Why not have the current owner go inquire before you buy and convert the lease to a near term expiration if the tenant does agree?  

There is an office that has a closet and could convert into a 5th bedroom with the addition of a window.  

You will likely find a lot of illegal "3" families homes with finished basements: Union City used to have a forgiveness program where you could legalize your basement without penalty (so, convert to a 3 family--my neighbor did this), but the cost of doing this is quite high, and I'm not sure if that program has continued.

I'd like to convert my first multiplex to a full rental unit as fast as possible and invest in additional properties.

I know postcards are cheaper but do they convert? 

I've got connections as far as trim carpenters to convert an office to finer finishes, however due to the buildings age not sure if you could get it of to an "A" grade status or not....the #'s probably just would not make sense at this point.