For motor and mechanical parts, go for 72-75. The motor and drivetrain changed quite a lot in 76. 72-77 will have pretty much all the same inside and outside trim except the front bumper and valance are different. (77 and newer tailights have amber turn signals if you'd like a change...) 78-80 inside parts will fit, but the gauges, dash vents, and glove box door are a little different looking style-wise.
If you look on the main
http://www.luvtruck.com site under info there is a nice how to on converting your front brakes to discs if you can find the parts. You'd be looking for a 76-80 donor truck, non-4WD.
Isuzu made trucks during the LUV years, but you don't seem them in the USA, so you're just looking pretty much for LUVs. A lot of the parts from 81 LUVs and later year Isuzu PUPs (which were sold in the states) are not a very good fit to 75 and older LUVs, so they would not be much help to ya.