There was a special edition of the Caballero called the Diablo. It had a picture of a demon on the front hood, reminding some people of the Pontiac Trans Am "chicken" front hood.
The base model of this car had a V6 engine, but engine options ranged all the way up to a 5.7L 350 V8. The automatic transmission was optional, but most cars were ordered with it. The Diablo was mostly a cosmetic stripe and decal package, not unlike the El Camino Conquista.
By 1980, emissions laws pressured GM to sell most Caballeros with a Oldsmobile V6 Diesel engine. It was only sold with the 3 speed automatic, and to say the engine was unreliable would be a huge understatement. The engine was so terrible, it ruined the reputation of diesel engines in the U.S. market for over 20 years. It wasn't until recent concerns over mileage and gas prices that diesels started to make a comeback in the consumer car market. Over 1 million of these engines were sold on GM cars such as the El Camino and Caballero.
The Diablo package was discontinued by the 1981 model year, as the model began its slow decline towards its eventually death in 1987. The El Camino was also suffering during this period, but at least it received some efforts to revive the brand. The Choo-Choo was an SS conversion available on the El Camino in 1983 with a L69 V8 engine. Unfortunately, no comparable package was ever offered on the Caballero.
The Caballero is considerably rarer than the El Camino. They are becoming more and more highly sought-after in the collector car market. El Camino fans consider the Caballero to be one of their own, albeit much rarer, so if you're planning on buying a Caballero or restoring one you should feel comfortable knowing that there are thousands of loyal car junkies out there willing to offer you help and advice. Like I said, these cars are rather rare, but they can be found. The first place I would look is eBay. It's the easiest place to search, and prices are generally lower than a car auction or even the general used market. Be aware of the diesel engine issues I mentioned before. Also consider that the later years are often sold for much less than the models from the 1970's, even though in many cases they were produced in lower quantities. The 1987 model, which was the last, sold only 1,882. That could mean that it will be highly valued in the collector car market in the future.