K10 to k20

From where or who could tell be the technical difference between Suburban (1980) k 10 and k20 version?

Is it only brakes or something else?

And from where I could get manufacturer information about specifications and RepairShopManual (orginal) from Suburban 1980?

The K20 would probably have heavyduty emissions, bigger brakes stonger leaf springs bigger wheels/tires, heavy duty rearend, lower rear gearing. This is just the stuff of the top of my head. As for the manual check your local library.

couldn't you just identify major differences from the VIN

The K20 will have a turbo 400 tranny instead of a turbo 350, The frame on the K20 is heavier duty, The rear diff is bigger, the front diff may or may not be different, The transfer case is most likely different, The front and rear springs are different and the brakes are different. Basically the only thing the same is the body.