If your company owned a specific equipment, it will be easier to evaluate since you can verify the maintenance history of that equipment. You can add the cost for a mechanic, parts replacement and maintenance the equipment will need during the period of your project. From that analysis you made, you can assess if it will be better to purchase a new equipment rather than to use an old one because of the incremental cost due to maintenance and parts replacement. Purchasing new equipment will always make your project cost proposal high. What we usually do is that even though we purchased a new equipment for that project we still use the rental rates on our estimate since we foresee that we can use it to our other future projects. This will be a great advantage since brand new will be less prone to breakdowns that can cause delays on the project.