Rule number one when looking for one of these, age and mileage mean little, condition means everything.
I have seen some lovely 82-85 K5's that were in nearly new condition, and have seen 89-91's that were trash.
Currently I am driving an 89 with the 5.7 TBI thats very nice, not without a few issues but at this age, one expects some little issues.
I am not a TBI freek, I love the 6.2 diesels myself and especially the stellar fuel economy that they can get. 24 hwy is easily in reach.
Nice thing is that from 82 on you can mix and match parts pretty easily.
You can swap the core support out and have an 85 truck look like a 91.
The seat mounting was changed in 88 I believe and the bucket seats are slightly different.
These have got to be one of the best of the small 4x4 trucks ever built.
The GMT400 series two door Tahoe/Yukon was nice , but not the same as the K5.
If need be, scrounge up a couple rigs to pick parts from then add such things as the front doors off an 88-91 Burb with the electric mirrors to have a really cool rig.
You can swap in axles from a 2500 truck along with the 8 lug wheels and have bullet proof drive train.
Install a TH400 tranny and the NP 241 T case too just to make it even better.
The K5 Blazer/Jimmy is a truely bolt together mix and match truck.
If you want a behemoth of an engine, you can drop in a 502 crate engine that will send your little 10 bolt rearend to the trash post haste.
Most everything on a pickup from the front doors forward will bolt right onto a K5
I have seen some very interesting K5 trucks.
There is a local fellow that has a 91 K5 with a DMAX/Allison in it.
I have seen the newer 6.0 small blocks installed as well as all manner of other iron fit under the hood.
There is even one character here that stuffed a 7.3 PowerStroke in his K5
If you can dream it up, its likely been done.
Have fun
Snowy