I think the real answer to this question comes back to the thread you started earlier, and at the end of the day the truck still should be inspected by a professional technician on a yearly basis. As long at it is in fit condition, it can be as old as you want it to be. I have always found Toyota 5 series trucks to be very easy to maintain and repair, and find parts for, but whomever works on this truck must be trained and educated on how to inspect and maintain the upright portion of the truck (mast) so a dangerous condition can be recognised and corrected before an accident occurs. Chain stretch, cracking and pin turning, mast bearing damage and clearances, mast channel spread and weld condition, mast support bushing condition, mast adjustment, and fork/carriage condition are all very important in keeping your truck safe and specs can vary from one truck to another. I would worry much more about whether you have a competent person to keep the mast tip-top rather than keeping the engine running. A running truck with a broken mast is much more dangerous than a dead truck with any mast. I know you stated the hours will be low but by your description of the jobsite they will be very hard hours and damage to the mast is likely in that environment. Pick a truck with good ground clearance and foam filled or solid softy tires, a valid mast inspection, and stay up on those inspections. Good Luck