time and material roofing contract

I am trying to take 1 building of 4 units that have flat and shingle roofing that have had many attempts over the last 45 years to solve a flat roof problem. My approach is to tear the roofing materials on the flat roof down to decking. Then i want to study all potential solutions on how to create a better permit solution we have had over the years. Normal roof have failed many times. I am a licensed contractor but not a roofing contractor and live and set on our HOA board. I need to write a contract between 2 roofing companies one is our consultant and the other is a certified roofing contractor for GAF materials. I need a contract that allow me to control a time and material plus allowing me to purchase all materials to the potential solutions. This test building I may do 2 different solutions to the flat roof that the purpose of the t&m and I want to do t&m so i can keep track of real expenses as we have 22 other building to do and when i set the spec out i will hire 2 or more roofing contractor assigning them a building but following our specs. So its important that this first building allows me to control the job and its solutions that our roofing consultat and the vendor and myself agree to try on this test building.