I could put a down payment for a conventional (20-25%) 550k duplex, but I feel it could hurt my chances of buying as many duplexes in a 5 year period (for example) if I go conventional VS an FHA at 3.5%. 

You need to minimize upfront expenses because 100% of your investment is at risk and lost if the lot is not buildable.My favorite strategy is to locate the property and tie it up for usually no more than $100 for a period of 8-12 months, analyze it myself and if I believe it to have a chance at being buildable I create a permitting plan and market the property to other investors/builders willing to take on the permitting risk.  

@Arun Philipnot sure if i understand your question.if you don't have any cash, period, you shouldn't be buying any more properties. 

Can you carry the property for the construction period?

My understanding is in Jersey, the owner has a set period of time where they can force the property to go to auction.

776 acres on the TN River - PERIOD How did you find this deal and how did you negotiate it?

That is amazing -Where did you manage to double the value on an 8 unit in a short period of time 

This long vacancy period can result in significant loss of income and added holding costs.

You will want to consult with a lawyer.In regards to your last two questions here's some background on what makes a contract valid and enforceable:Offer: A promise to do or not do something within a certain time period Acceptance: A clear and direct statement that all terms and responsibilities are accepted Consideration: A legal and adequate inducement given in exchange for the promise Capacity: The parties to the contract must have the legal capacity and competency to contract Awareness: A party must be aware of the contract Legality: The contract must be legal in the jurisdiction it will be operating in***Not legal advice*** Steps I would take: -Review contract in detail to understand each counterparties responsibilities (if you are not aware)- Email a copy of the lease to each of the tenants listed on the lease and reiterate lease expectations for termination notice, rent payment etc. - If the lease was signed via Docusign (other esign may have same ability) - you can go to the envelope, go to history and view the IP addresses for each action taken.

We arranged closing during a slower period of time & had him leave everything (furniture, inventory, guides, etc.) literally bought it & could start taking booking the same day.