You can usually find a nice D series on ebay in the range of $50-$100. I look at the condition of the Dive/ Predive switch to gauge general condition (it should have square corners and not be ground down), and look closely at the rubber shell. If the rubber shows signs of deterioration or tears or cracks stay away. The rubber should still maintain a smooth, almost glossy pebble finish. If the rubber is no good, the reg is no good.
The d300 and d350 all come with metal orifices. A d400 made at the latter end of its production run came with a plastic orifice. Metals are much more sought after, but the plastic orifice model can also breathe OK, IMO. You probably won't know which kind you're getting til you take a very close look. The seller will probably not know.
