Hydraulic tank materials

I am a Mechanical Eng Tech in Westlock Alberta Canada. We design industrial tank based units for the oil field. Most if not all of our trucks have some sort of hydraulic system. Due to demand we are trying to develop our own hydraulics department and keep everything in house. The solution we're looking for is what material to use and what process should be taken during assembly to ensure no contamination (within the reservoir). We've entertained ideas of anything from stainless steel, aluminum, and possibly pickling steel. What material will provide the best long term results in all aspects (contamination prevention and fabrication ease). As for fabrication when installing baffles what should be done to prevent the weldment from being a potential contaminant. In summary I'm looking for the best material for the job and a process that will result in a long lasting product.