That isn't high mileage if the truck has been taken care of my friend! For maintenance- hopefully you went over the truck before buying and looked for oil leaks, turbo damage, and all the usual stuff (brakes, ball joints, etc). I would start by pulling the intake off and checking the turbo wheel for signs of dusting- the blades on the compressor wheel should be straight and smooth- any jagged edges or missing pieces of blade means its time for a turbo rebuild. Then shine a flashlight in the valley of the motor and look closely for any wet spots (it should be dry in a perfect world). If you find puddles, try to soak up what you can, then give the engine a hose-down with Simple Green (cover powersteering pump and intake (if open element) with a plastic bag, spray on Simple Green, let soak, rinse off, then start the truck and let it run to help dry up the water. Remove the bags before you start it!
) Don't spray cold water on a hot engine either! Doing this will allow you to pin-point the location of the leak. Oil is a personal choice, I use Shell Rotella 15/40- Rotella and Delo are the preferred Dino oil, Rotella Synth or Schaeffer's seem to be the preferred synthetics. Use Motorcraft oil filters ($10 at Walmart, part number FL-1995).
As for mods- what are your goals or plans? The common mods are gauges (boost, pyro, and trans temp), a good intake kit (I like the DIY 6637), 4" turbo back exhaust, a good tuner (DP Tuner, Tony Wildman, or PHP) and trans mods (if you have an auto) to help keep the trans happy.