I would never think of it as an assignment fee because your fee is wholly dependent on what you write your contract for, your estimation of ARV and repairs, your buyers' list, your marketing, and your reputation.

All parts of real estate run on reputation and deal-finding.

If you're buying through a real estate agent that may be a decent place to ask for a reputable contractor. 

People were still manufacturing buggy whips while the Model A was being churned out.  

This will help you build a reputation and a following.

Build trust and reputation.

Definitely speak with a reputable real estate attorney, regardless of how you mange to secure the mortgage.

Your duplex will appraise the same as any other stick built duplex, it'll insure the same, etc. it is NOT a mobile/manufactured home, but you're going to have your construction loan for a lot shorter period of time which means less interest you pay and the faster you get in a tenant and start that cash flow.Importantly.. you should come in WAY below a $250 sqft price.. here in Houston I have a modular builder that starts at $88 sqft and with the improvements i am requesting just about hits $100.

@Marco Morkous - I would first call a reputable plumber to run a camera down the main line, and if all clear, then your issue is from the city sewer main.

Preferably from a reputable real estate investor who focuses mostly or entirely on fix and flips.