They did have their ups and downs but if you kept up on maintenance and take care of them they will run forever. And with the low miles this K5 you're talking about has, it seems like it's sure to be a good motor. I've owned two 6.2L Diesel trucks and a 6.5 and they have all done the job just fine. My 82 GMC 6.2 that I just sold would average 21-23 MPG all day long and usually 17 or so in town. They weren't the most powerful but that's nothing a turbo can't fix. The K5 diesels usually saw a little better MPG than the trucks but were given the "weany" injector pump and usually are underpowered, but you can turn up the pump a little bit to give it some more get up and go. I'm sort of obsessed with these motors actually and know way too much about them but oh well haha.