for my long drives with the 300TD i tried to always have:

-heater hose
-2x of each type of filter (primary fuel, secondary fuel, oil, air)
-fuel line
-vacuum lines
-more than the engine's capacity worth of each fluid (oil, coolant, brake, p/s)
-glow plugs
-more than a full set of fuses
-stuff to do wiring (wire, solder, torch, crimps, stripper, etc)
-floor jack + jack stands
-2 spare bundts with tires on em
-multiple spares of all bulbs
-2 of every belt type (ac, ps, alt/water)
-spare SLS high pressure line
-large size fix-a-flat
-couple 4x4 blocks
-couple 2x4s
-spare battery (charged)
-4x 5gal military fuel cans (the awesome scepter ones you cant buy anymore) full
-2 sets of jumper cables (1 set wont start a dead w123)
-MREs (military food rations)
-blankets + pillows


with that, the only times i ever had to call for a tow were when i blew a head gasket, and when i fully seized an engine.