Ok, you mentioned things that are junk and will give you trouble. That's a easier list, though not short.
1. Wiring to the headlights and other lighting is too small and will melt the wiring and the plug at the headlight switch. Someone mentioned the relay mod for the headlights, and you might need a relay for the running lights also if you pull the trailer or have a camper.
2. The fuel sending units in the fuel tanks are junk. The "shower head" pickup breaks off and you run out of fuel around 1/4 tank, and the resistance thing that works the gauge wears out. And new sending units are very expensive and hard to find.
3. The fuel switching valve that switches the tanks are getting old and will usually fail. The parts stores sell a generic one that needs some fabbing to hook up, or you can find a Ford one that is very expensive. Or people have their own methods to switch the tanks over, there are many ways to do it.
4. The fill system/necks for these trucks stink. The first time you go to the station to put fuel in, you will find this out. Do a search on tank venting, it's a common problem.
5. The instrument cluster lights on the 86-dwn trucks are very inadequate and dim. They have colored lenses in the cluster that cloud over when they get old, making the gauges very hard to see.
6. The fan clutches wear out. That's not a problem, they have had a long life, but the new replacement ones are junk. There are threads on here and there are several different ways to fix this.
7. The front brake calipers like to stick, and the slides that the calipers ride in like to rust and stick. You will probably be messing with this in the near future, just make sure you lube the caliper mounts and slides with never seize or something equivalent.
8. The rear parking brake cables rust and seize up. You will find they are very cheap at the auto parts stores, because they sell so many. They are a bit of a pain to replace, just cross your fingers and hope you have the rearend where your rear drum just slides off. If you have the Dana where the drum is behind the axle flange, then you have a job ahead of you.
9. The brake lines like to rust in two. Common places are about midway behind the side fuel tank, and right where they hook to the rubber lines up front. This happens at the worst time possible, so look at them and see if they look bad.
10. Look at the seam above the doors on the outside from the drip rail to the back upper corners of the cab. This seam likes to open up, and water leaks down the inside cab corners and rusts the cab corners out.
11. The glowplug controller likes to fail, and ruins your glowplugs. I like the manual switch conversion myself, but some people like the automatic system, and upgrade it to the later 7.3 system(if you call that a upgrade).
12. Return lines and o-rings on the injectors have been mentioned already. This is a good system, it's just getting old and time for replacement.
I am sure there are some trouble spots I have missed. Are you sure you still want to buy one of these things?
Theres so many things you can do to older pickups to improve them you cant really write it all down. Plus it depends on what you want it to do, are you towing or after mileage?
