The Conshelf series 1st stages are all the same internally, and parts are still available.... I have and use everything from a Conshelf VI, to a Conshelf SE2 to a Conshelf 21. I prefer the older metal Conshelf seconds, the the plastic ones are functionally the same (most parts are interchangeable too). The 22 is the second generation of plastic seconds, and not near as prone to cracking by the inlet port as the first gen.
Conshelf's are also a pretty good intro to self service... very little in the way of special tools needed and standard replacement parts (service kits) are easily sourced.
Respectfully,
James