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Dont be fooled into thinking these things are complicated pieces of equiptment, they are not. Get what you can afford, have it serviced regularly (better you learn to do it) and they will last a lifetime. Dont let age of a used unit scare you off, there is some great stuff out there.
Old regs scare the dickens out of me.... or at least anything older then my 1958 DA Aquamaster.
Old Sherwoods are a good choice, rock solid and easy to service. They may not be the best performers around but odds are they will still be diving when the new whiz bang regs are piles of plastic junk.