I did some informal training and decided that I was much happier being a chickenshit with both feet on the ground than a superhero with two legs dangling in the air.
You need to be able to tie a bowline one-handed without looking. You need your knots and your knowledge of redundant gear and lines. You need to be an anal retentive asshole about rope and metal because when that stuff hits the end of its useful life, it gets destroyed. You need to use it often because practice is proficiency and you need a budget that will allow you to replace it.
"Some rappelling on the last day," is not good enough. All that will accomplish is to give you a taste of what it feels like and what you have to master. If it's for you, you can meet some very cool people and do some very cool things playing with carabiners.
DON'T SKIMP. The fifty bucks in your wallet that you saved on gear will just make you fall faster.
I avoided confined space training and I don't regret it. We had 3,000 gallon nitric acid tanks at one place where I worked and the engineers had to have confined space certification and follow all the protocols every time something had to be serviced in one of the tanks. They recertified every year and typically entered the tanks semi-annually, but they were engineers and the whole system was their responsibility. The head of the plant was also certified, though he didn't do the actual work. The two engineers expected that if there was ever a problem, they would be dead before the fire department could get there and rescue them.
This was a nice, innocuous warehouse looking thing in the wealthy and densely populated town of Delray Beach, Florida. Aside from the NFPA placards on the building, you'd never know that we had 6,000 or so gallons of nitric acid waiting to murder you if something went wrong.
My current employer is in tornado country and has an ammonia based refrigeration system. I honestly thought about quitting during the initial training when they told us the procedures for that.
The difference between volunteer firefighters and engineers who know they will die if something goes wrong in the confined space is that the engineers spent forty hours a week with the nitric acid system and understood it. Speaking as a former volunteer, I had other things to do. My understanding of the hazardous materials and confined spaces in town was limited by my need to earn a living elsewhere and have a life.
This is another one that, if you're going to do it as a volunteer, you need to drill a lot. Fortunately, the haz-mat places in your AO should have a much more sophisticated understanding of their dangers than they did in my day. They should be willing to work with you on their particular systems. I read somewhere that railroad boxcars are good for this because they have a hatch on top that's totally inadequate for rescue. You can put piles of stuff inside to disorient your rescuers and if somebody freaks out, you can open the side door and go get him. All you need now is a train.
Good luck and IMO, the best thing you can do is be the guy that everybody hates when he designs the drills. You'll save lives that way. See: Rick Rescorla.