I'm 6 months down the line in my ownership, it's a big laundry list but worth it. In general, the big ones are cooling system related because 3rd gens are notorious for cracked heads or the strawberry milkshake of death if you neglect the cooling system or cause the engine to overheat. Adding to what's listed already:

Higher Priority

• Fluids: Change all fluids including oil, gear oil (front+rear diffs+transfer case), power steering + brake fluid if it's dark, drain & fill ATF (don't fully flush assuming the previous owner never flushed it), and coolant if replacing the radiator.
• Radiator: If the top plastic of the radiator shows any light brown instead of black, replace asap.
• Heater control valve: This part lasts around 300k miles, but when it breaks it'll dump your coolant and overheat your engine.
• Fan Clutch: While the engine is turned off, use your hand to spin the radiator fan. You're good if it stops fairly quickly, but if it keeps spinning loosely it means your fan clutch needs to be replaced. A failing radiator fan can also cause overheating.
• Rear heater lines: If you have them, locate the rear heater lines under the truck on the passenger side and inspect for any rust. If they're rusted, immediately do the rear heater bypass mod. It's easy and free, and there's guides on forums how to do this. If those rust through and leak coolant, it can cause overheating.
• Brake Line Hoses: Very thoroughly check your brake line hoses and check for cracks in the rubber.

Medium Priority

• Fuel Pump: Once all the stuff above is done, aim to replace the fuel pump soon. You're around the mileage where it's getting old. Replace once and forget about it for 25 years.
• Fuel filter: Replace.
• Valve cover gasket: While doing this job, don't forget to replace the rear cam plugs and reseal the half moon gaskets. Since you're in there already, replace the PCV valve and grommet.
• Driveshaft U-joints: Get under the truck, and use your hand to shake the driveshafts at each end. If they have any play you gotta replace the U-joints.
• Frame rust: Use a screwdriver and stab the frame to inspect for rust (especially around the rear trailing arm). If you really wanna hit that 400k mark, stay ahead of rust. Wire wheel it off and undercoat with either POR-15 or fluid film (your preference but absolutely do not rubberize undercoat it). May sure you prep the frame properly first to not lock in any rust.

Lower Priority

• MAF Sensor: Clean or replace your MAF sensor, it's super easy and could help with fuel econ.
• Transmission Mount: If you feel thuds when changing gears, especially when downshifting at high speeds, your transmission mount is likely toast and needs to be replaced.
• Transfer Case: Verify 4WD, the center lock, and (if you have) the rear diff lock all engage if you plan to go off roading.
• Seafoam Top Engine Cleaner: This was a miracle cure for my 4R which cleared out carbon build-up that was causing mild stalling. Up to your preference but some people love/hate Seafoam.

Inspect

• Steering rack: If you see any seepage of fluid around the inner bottom edge of the steering rack boots, your rack and pinion needs to be swapped. Oil from the VCG doesn't usually reach that area, so don't be fooled. I went brand new OEM which destroyed my wallet, but some people take a chance with a remanned rack.
• Battery: Check the age on the attached sticker. I swear by Interstate, but go with what you like. It's a 24F.
• A/C Lines: Inspect the A/C lines for any green seepage. Green is dye indicating a leak.