Tensile Testing System

Maybe you could just buy one.

Used TENSILE TESTERS - TESTING EQUIPMENT: Used Machinery

You need to design the system using hydraulics. Hydraulics offer the highest level of precision for tensile testing machines, which is why they all use hydraulics. You will find that a rack and pinion style design will not be accurate and repeatable due to varying forces and frictions which you need to overcome. You can easily control an exact force at an exact rate of speed as slow or fast as you want using hydraulics.

You can use an electric motor (if electricity is available) or a gasoline hydraulic power pack, valves, filter, cylinder, etc to make it mobile.

I think you are attempting to create something which is overcomplicated, and under the level of accuracy needed for a testing machine.

"Make things as simple as possible, but no simpler" -Einstein

As far as you actual question about the DC motor, yes many are available, you can use DC or AC as long as you get a good motor speed controller, and choose a motor that is rated to run at a reduced speed without the fear of overheating. You should be able to find ample information with a quick and simple google search of "motor speed controllers" or "variable frequency drives". If you are on an extreme budget, and the motor is small enough (you never stated the power requirement for the motor) you could use a simple PWM (pulse width modulation) to vary the speed of the DC motor.

But in the end, your contraption is highly overcomplicated, and highly susceptible to poor repeatability over time.

This was all discussed in http://cr4.globalspec.com/thread/68644 the advice then was NOT to go with a rack and pinion.