First thing to do is to drain and replace ALL of the fluids. This includes the transmission, differential(s), transfer case, coolant, power steering and brake fluids. Install a 16 psi radiator cap to replace the 20 psi stock cap. This will likely reduce the odds of the radiator failing due to the system pressure being too high. I did this many years ago and have not had any radiator issues since. Change the transmission fluid and filter along with draining and refilling the transfer case religiously every 60k miles.
The 5R55 transmissions in the V6 4th gen Explorers are not the most reliable. The best way to keep them going is to do diligent maintenance on them. A high percentage of the 4.0L SOHC engines eventually have issues with the timing chains and guides largely due to poor engine oil maintenance. Changning the engine oil and filter EVERY 5k miles with full synthetic oil will go a long way in keeping this problem from occurring for as long as possible. There are other things that can be done to increase the life of the chains and guides but I will leave the discussion of this to the more knowledgeable members here. My experience is with the V8 models.
If you live in an area where the roads are salted, then have your vehicle treated with an oil or wax based liquid rust preventor. Do no use a rubber or hard undercoat as this will just accelerate the rusting process. You will need to reapply the treatment either annually or you might get by with doing it every two years. If you don't then rust will consume your vehicle quicker than you would like.
Lastly, pay attention to when strange noises, rattles, vibrations, smells, codes, etc. occur. Don't put off repairs, if possible, because a neglected repair can cause other repairs to be needed. Typical problems as the miles rack up are wheel bearings, bushings, ball joints, occasional electrical gremlins, sensors, etc. I bought my Mountaineer six years ago with a little over 100k miles on it and did all the maintenance items I listed in the first 5k miles. I bought it knowing the radiator was leaking and needed replaced. It now has 152k miles and in that time I replaced the rotors, replaced a caliper, the throttle body, the Y coolant hose on the passenger side (common problem), front and rear sway bar end links, will have to replace the sway bar bushings this spring, the AC compressor and maybe a few smaller repairs I am forgetting.
This might sound like a lot of repairs but I knew there were going to be items failing as the miles rack up which will occur when buying a vehicle with over 100k miles on it. However, the really expensive repairs did not occur and this is where, IMO, the maintenance referenced in the first paragraph pays off. The 4.6L 3V engine runs as good as new, as does the transmission. There is no differential whine, the transfer case is good, the cooling system has been reliable other than the Y hose and the power steering rack has not had any issues. If I drive this vehicle long enough to reach 200k miles then I'll repeat the maintenance items needed again.
