The K5 - what year and is it the 2 door or 4 door version?
Oh , and to the original poster, tires and brakes first, of course. Then suspension. Next up is locking it and gearing it right(this can also happen at the time you do tires of course). Beyond that, armor and sliders is often more than enough. You don't have to go silly with these trucks. SAS/Lifting/dual-range/etc are all advanced stuff to be done later - plenty of people wheel for the first few years or even decades without doing that. My 87 is pretty much stock other than some of the aforementioned mods and about a "2 inch lift" from new springs a few years back.(ie - rides stock height, about what a new Tacoma does vs sagging like 95% of what you see out there)
It handles trails, snow, and all of the basic non rock crawling stuff fine if you go slow.(that's the hard part for me - I'm not terribly patient)
