I’m in contract on a mobile home park that is eligible for a USDA loan.

My way of thinking was I get the house under contract, take out the loan to make the mortgage payments, and then on the resell of property ideally I'd want a large enough profit to where I can repay my loan and still make a profit. 

Have under contract a vacant lot where I will have a small house and a "granny cottage" behind.

By doing so, you are inviting a visit to court where you must prove that you are not doing a Land Contract (and you will lose). 

We are in a contract to sell a two family in North Jersey and should free up about $525k for a 1031.

Each state has different contract laws so you have to use one that is legal in your state. 

One way or another you are going to pay for your "tuition" - either a. being scammed by gurus hoping they will teach you the "secrets" on how to become a millionaire in your first year/month/property/flip/deal, or b. while doing mistakes in your first projects/deals (but at least you get something out of it and is the best form of education)  or c. putting a lot of time, effort and resources in learning about real estate investing industry (and that includes marketing, financing, negotiation, contracting/legal, taxation, hiring, project/people/property management, etc.).

-What type of contract should I use to write up the terms?

What is the right way to do this so that the contract is with me and not the previous owners?

We recently had a hard money lender back out of a land purchase deal we had under contract a week before closing date.