Watch some Alec Peirce "Scuba Tech Tips" on YouTube to get an idea of what is important and what are extras.
For BCD's, besides the bladder being able to hold air, make sure the purge valves work and the power inflator works. To test the power inflator, you'll need a tank, which likely you won't have. Hopefully a nearby Dive Shop will be accommodating. What you want to make sure is that the bladder doesn't inflate without you pressing the button the power inflator; that would be bad during a dive!
For regs, you can find new regs that are priced moderately, but you won't get all the fancy features. The videos above will tell you what's relevant.