Good idea before buying one is to know what you want out of one. I knew these were rare and potentially somewhat valuable as a collector vehicle, but I still wanted one mostly for actual camping. That's a slight mistake, because now I think I'd be wrecking the value of mine by altering it to suit my tastes - engine, tranny, back door, interior. 20/20 hindsight says that as good of original condition as mine is in, I would have been better off getting a non-original early '80s diesel Blazer or Jimmy that was modded to be a good cruiser with the extra weight on the back, and then I could add a modernized orphaned Chalet shell. Same spiffy-looking uncommon vehicle better suited to what I want, with no worries of messing up a rare old camper.
When buying one, consider how much or little you want to spend to fix it up the way you like. Some are around for $1000, but you may end up spending $8000 just to get it looking as good as mine, which I'll sell for $9500. Is that worth the effort for the low price? Guys who like restoration work don't mind that, others end up regretting buying the first junker they spot.
GM trucks rust big time, so determine how much repair work you can tolerate. Check the cab floor because rain might have collected into the area behind the doors and ultimately rots out the floor. Even mine as an AZ truck has rust spots in the corners of the fenders, an unavoidable condition, but not too bad to fix.
Check how long the vehicle has been sitting. Longterm storage means the rubber stuff will not be in good shape, that's the worst part about mine. Most likely, the vinyl sections of the pop up roof will be tugging loose from the roof, I've only seen two that didn't have that problem. Repair means either finding a way to heat / stretch / and re-staple the loose corner(s) back on, or a complete removal and replacement using the old bits as a pattern, if the original vinyl is hopeless.
More than likely, the LPG tank has the obsolete overflow valve. A new DOT-approved tank is less than two hundred bucks, and a new gas regulator is $40-ish. The cooktop stove is fairly bulletproof, but you are better off if the seller can prove that the fridge works. One guy said really rough bumpy driving broke up the ammonia crystals in his fridge's closed system, and it worked fine after that, after being in storage for years. That's one way of "shaking up" the fridge, I guess.
The back door latches are fragile, expect to see it broken or replaced by any variety of other latches. Chinook wasn't especially good about making fine quality doors, check to see if the whole thing is about to fall apart.
There's various other things, I suppose, much like any other '70s vintage camper. The main thing is to consider what kind of inherent problems you are willing to put up with, or that you or someone else can fix.