Well it more depends on how well maintained the previous owner kept the vehicle. Stuff like servicing the transmission, miles and rust would be my main concerns. Transmission being the hardest to see any problems. Pull the transmission dipstick out and check the fluid. Should be bright red. If it's dark and stinks with a burnt smell then the tranny has not been serviced and could be close to failure. See what kind of service records the seller can provide. look around a little on the property (if your at the owners home) and look for things like larger heavy travel trailers or car trailers the the K5 may have been pulling. Pull the oil cap off and see if there is a larger amount of deposits visible indicating infrequent oil changes. Some deposits and black varnish is to be expected with a higher millage motor but if it looks like dried black mud coating the inside of valve cover it's not been trated all that well. If the seller has a good set of recods showing good maintance then it's probably not going to have any major problems unless it has high miles. Take it to a GOOD repair shop and have them do a compression check.
THe only known weakness these trucks have are transmission from lack of maintance or abuse from large tires and wrong gearing and the rear axle is not the greatest for off road abuse. Street with no towing they usually hold up ok. Look for signs of heavy trail use by looking for mud packed into the front cab mounts, the tail ban (part the the tail gate bolts too, check that for rust as well. Look at the bumps stops for the axles. Lots of wear and tear on those and it's been off roaded a fair amount. Front get regular wear but the rears seldom hit the bump stops on the street. Check the springs for broken leafs. Thats another sign of hard off road use.
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