The two-wheel drive Chevy models had the C in their model name, while the four-wheel drive had the K. The GMC models did not have the C, but did use the K for the 4WD trucks. The numbers were different, so let me explain. The Chevy C10 pickup becomes the GMC 1000. The Chevy K30 (4WD, three-quarter ton) becomes the GMC K2500.
The Sierra name was later adopted by GMC for these trucks, but the letter and numbering structure of the names remained.
For the first few years of this truck, there were only two trim levels: base and Custom. The top engine available was a 160 horsepower V8. That was boosted to a robust 220 hp in 1965, the first year air conditioning was made available as an option.
These trucks have become far more powerful as the decades passed. They are now heavy-duty workhorses, seen on construction sites and farms across America. A Vortec 6000 making 345 horsepower is now sold on the SS extended cab trucks.
With a 45-year heritage, the GMC Sierra truck is one of the most common on the roads today. You can find plenty of them on the used truck market, along with thousands of owners out there who are more than willing to help you find the right part for any fix or modification you might be considering. The cheapest parts I can find are sold on eBay. You can also buy the whole truck on there. With modern services like CarFax history reports, it's not nearly as daring to buy a car off an online auction as you might think.