are you buying from a dealer? that's what i did. i test drove a few of them and started figuring out which ones would need the most to make it a daily driver. for example one sounded like it ran a little better than another, but on the road it was like i was in a fight to keep it in my lane. i definitely agree with the post above about glow plug operation! the day after i got my m1009 a glow plug was shot and it wouldn't start. i had to replace all of them for good measure.
check everything you can without taking things apart ie. wiper operation, all lights, speedometer/gauges, look underneath at the engine and axles for leaks, feel brake operation if you can test drive, trans shifting(to me this one is a matter of how does it feel, smooth, rough, but these are old trucks too), overall rust condition.
i don't know how mechanicallly inclined you are but this was my first diesel and truck(most of my previous cars were vw's), so there's definitely a lot you can learn from this site as these aren't exactly like civy trucks. use the search function and ask for help and in my experience on here so far people will help.