My thoughts. The ZX2 is probably suffering from an intake leak somewhere or a dirty IAC. If the CEL is on, and it should be, the codes can help steer you to a solution. If the CEL is not on, maybe a dirty throttle body is your problem. The ECM may not see a high idle as a problem if it thinks your foot is causing it. A can of throttle body cleaner (NOT CARB CLEANER!!) may be all you need.
As for the Caddy, a head gasket job at a shop will be pricey. Depending on the engine, it could run from $2,000 to $5000. Some shops may not offer to do the job. It is very labor intensive, requires sending the head(s) out to a machine shop, and takes up a lot of bay time, some of it idle until the parts come back.
However, if you have the inclination, the tools, and the time, it is a do-it-yourself project that could be rewarding. I did a Volvo BHG for my cousin. She was quoted $4500, and I did it for her in her driveway for less than $1000, but it took me 3 weekends to do it. First, start out with a good manual. The best is a factory shop manual, but a Haynes or Chiltons manual for around $20 with pictures to help guide you is a good option. They are typically designed for the DIYer and include details for a step-by-step breakdown of the entire engine. Plan on taking the head(s) to a machine shop to check for flatness and cracking, grind the valves, and replace the valve seals. Last time I did this, it cost me $300 to get the head tanked, shaved flat and re-seat 20 valves. This was pricey, because it was a Volvo DOHC. A Caddy head will be cheaper. Hint, the valve seals are generally part of a head gasket kit, so make sure you get that first. They range from $80 to $150, depending on engine.
Tips to offer include using masking tape and a sharpie to mark ALL WIRES AND VACUUM HOSES at both ends that need to be disconnected as you go. This makes reassembly a breeze. If the head bolts require a final torque angle, replace them with new before reassembly. These are torque-to-yeild bolts and cannot be re-used. Drain the coolant, but leave the engine oil. Change the engine oil before you try and start the car, so any contaminants that may have dripped down in the crankcase can be removed with the oil. If the oil is milky, plan on an oil change and coolant flush-n-fill about 100 miles after a successful repair to help clean out the rest of the mess.
Also, pay attention to things you can reach while doing this. Certain seals, hoses, and belts could probably stand to be replaced that may not be included in the kit. I’ve found it is best to replace this stuff when it is easier to get to, since I’m already down in there.