If I decide to purchase using a trust, do I have to form a business entity?
No
So all I need is a trust form right?
What is it you want to accomplish with the trust? It is a tool to do a job. It is not a one size fits all solution.
Entities can provide some asset protection and some tax benefits. Many beginners WAY over-do the entity thing. Unless you KNOW that you are going to do RE and succeed, I would do your first deal without an entity, and use the profits to form an entity for the second deal. As BLL correctly points out, LT are just a tool. Whether that tool is appropriate, and whether other tools such as entities are appropriate, depends on the facts - what you are doing and what you mean to accomplish with each tool. What are you doing and what are you looking for an entity or LT to accomplish?