Welcome to TDR MG. In my opinion, probably the 2 most important and effective modification you can and should do is to replace the lift pump with either an Air Dog or FASS.
http://www.vulcanperformance.com/AIRDOG-FP-150-fuel-system-p/a4spbd004.htm
https://www.thoroughbreddiesel.com/brands/fass/
I have the Air Dog 150 & big line kit on my truck and the Airtex with big line kit on the wife's truck which is stock. Both work well for the application they are installed on. There is not that much difference in the price between the Air Dog 150 and 100 so I went with the Air Dog 150.
Either of these is not a absolute requirement but they do offer superior protection to the VP-44 in that they ensure adequate fuel pressure and have a 2 micron spin on fuel filter and a spin on water separating fuel filter. If an Air Dog or FASS is not in your budget, relocating the lift pump to the frame below the fuel tank and replacing it with an Airtex 7153 fuel pump is an inexpensive but effective alternative. Give Eric a call at Vulcan Performance. He can go over everything you will need. He has the parts and kits for this:
http://www.vulcanperformance.com/Vulcan-Pusher-Pump-System-for-98-5-to-04-p/prsat.htm
http://www.vulcanperformance.com/Vulcan-Pusher-Pump-System-for-98-5-to-04-p/prs.htm
Even if you plan to keep the truck stock, in my opinion, this is one of the best and most important mods you can do. The other mod that is just as important is installing a set of gauges. At least install a fuel pressure gauge. If the truck will remain stock, a boost pressure and egt gauge although really nice are not absolutely necessary, however, I highly recommend installing them anyway. You can almost never have too much information about what is taking place under the hood while driving.
If you will be keeping the truck stock, the OEM air filter will work just fine. If not, I recommend the BHAF:
http://www.vulcanperformance.com/Big-Honkin-Air-Filter-BHAF-p/bhaf.htm
I suspect I may draw some objections but I advise you use PAPER filter ONLY. DO NOT install ANY of the cloth/oil high flow air filters on your truck.........EVER. They may flow more air but they also allow more dust and abrasive contaminants to enter the intake track of the engine and thus shorten the life of the turbo and piston rings/bores.
If your truck is like mine, I still had the original shocks on it up until about a year ago. I finally bite the bullet and purchased a set of Bilstein 4600 shocks on the wife's and my truck and the difference in ride was amazing especially on her truck which is a long bed.
It is unlikely but still a good possibility that you may have the original brakes on your truck. If you are not sure, inspect them and properly service them. Over the years, even though there may be lots of pad or shoe material remaining, corrosion and rust takes a huge toll and can cause the calipers to seize up. The brake fluid should be replaced about every 2 years but rarely does get replaced. After over 16 years, my truck had the original brake pads with about 85% of the pads remaining but they would overheat on long trips due to the calipers being seized up due to corrosion and rust. It took me about a gallon of fresh brake fluid to flush out the sytem properly when I serviced the brakes. I am VERY embarrassed to say that 16 years old brake fluid does not smell or even look like brake fluid any longer.
The other fluid that almost never gets changed is the power steering fluid. I vaccum out the power steering pump reservoir and refill with new fluid at each oil change to keep it half way fresh. The same applies to engine coolant. It should be replaced every 2 years but rarely done. I use Shell Extended Life Engine Coolant which is good for 6 years but I check it and vaccum out the radiator and refill with a gallon or 2 of fresh Shell ELC to ensure the coolant additive package is up to par.
Lastly and there are more mods than this is to re-route the engine crankcase breather and remove the puke bottle on the radiator that oils and mucks that area up and route an extension hose/pvc line down below the front axle. This will alleviate regularly having to clean up the mess from the OEM crank case vent hose and bottle.
These are just some of the basis modifications that come to mind with a used stock truck. I tried to refine this down so it was not TMI but I think it ended up being TMI anyway. Good luck with your new but used truck. Your friend must think VERY highly of you if he gave you this truck. That was NO small thank you gift.