Licensed Agent. Purchase question

[Agustin M.]

I'm a licensed real estate agent in Texas. Fairly new to both being licensed and investing. I already have one flip under my belt. I do have questions though. 

There are two off market properties that I wish to purchase. Off market I mean as far as they are not listed on the MLS. Do I have to use the TREC contract forms to purchase a property even though I'm acting as a principal? Or can I write up my own purchase contract? I'm going to be using cash for a quick close if that makes any difference.