Age and mileage are immaterial really, it is the condition of the vehicle that matters, and selecting the correct vehicle for the job.

A friend did the same rally in a Fourtrak, he drove it there and back, and as usual i ended up with the bulk of the vehicle prep work.
Begin with an older leaf sprung model, ensure the suspension is tip top, and replace all bushes with poly bushes for reliability, and fit good tyres such as Cooper's STT, these are incredibly durable and last without as many punctures as other premium brands, at much lower cost.

Replace all fluids, engine, gearbox, transfer case, and axle oils; as well as the coolant, thermostat, power steering fluid, and the brake fluid, ensure these all work properly, particularly the four wheel drive. Check all axle bearings, brakes, brake pipes and fuel lines; while the vehicle is in the air also check the steering, steering joints, propshaft UJ's, and grease the UJ's and steering ball joints.

Fit full underbody protection, use substantial steel plate, axle guards for the solid axles, and ensure all lines and pipes are protected fully, it may be prudent to replace brake lines with copper pipe, and the flexible hoses as a precaution.

Replace all engine hoses, drive belts and lubricate all hose clips with grease, and the throttle cable with engine oil.

Fit a FULL roll cage for protection, and refit the interior to carry all the spares and equipment you will need. Ensure everything is tied down well, and fit a mesh grille between the rear of the vehicle and the front seats for your protection.

Remember that these earlier vehicles require servicing every 3000 miles, so ensure you carry enough consumables to do this on these solid axle variants.

Research vehicle modifications done to other expedition equipped and modified vehicles to get as much information and ideas as possible.