Mostly general maintenance issues from the previous owner.
I have replaced the following:
Fuel filter, 3 fuel injectors, rear axle seals, all brakes, valve cover gaskets were leaking, radiator, all lubricants, lower ball joints, tie rod ends, spark plugs (needs dual ground NGK plugs only), plug wires, PCV valve. I'm sure I've missed a few.
That said, this truck runs like it is new, even with 160,000 miles. Many of these go well over 300,000 miles. Here are some special notes that I wish I had known about before buying this truck.
* Please be aware of the strawberry milkshake. It is a big problem, but completely preventable. Please note that the radiator is directly secured to the truck body, and not isolated with rubber like a typical car would be. The truck's stiff suspension will shake the radiator hard during its lifetime. The transmission fluid cooler, which is integrated into the radiator to maintain proper transmission temps, can break inside the radiator and mix with the coolant. The transmission will be ruined if this happens. Spend $120 and replace the radiator every 100k miles. You don't have to do it, but if it fails, it will cost you thousands $$.
* Also, these trucks have a known lower ball joint failure issue, which will also be catastrophic if it fails. The design on these has the stud pointing down with constant separating force applied to it. Newer T4r have fixed it by orienting the stud upward. Again, replace the lower ball joint every 100k miles. It won't give you warning before failure, and is difficult to test for slop.
* The rear axle seals were redesigned in 2003, and are difficult to get to seal properly. Make sure you check the rear brakes for signs of gear oil leakage. If you can't do this yourself, only let a Toyota dealer do the repair and get a warranty on their work. It sounds worse than it is, but otherwise these are great trucks. I speak from experience with my new to me '99 4runner with 160,000 miles on it.
* You can upgrade the front brakes to the 199mm Tundra calipers and rotors if you have 16" wheels. The bigger brakes will get rid of the shake when applying the brakes. These are the biggest weakness of this truck in my opinion.