Best used budget Stax system?

Really curious on the fuss for electrostatic ear-speakers. I have a pair of electret headphones (Stax SR-30) just to dip my toe in. All this talk about how much better true electrostatics are makes me curious. A few questions:
  • Does anyone have any thoughts on the best used Stax headphones?
  • Is there a better deal out there versus the SL-300 as an entry set of headphones?
  • Is there a difference in the sound of normal vs. pro bias electrostatics?
  • Any thoughts on the (now ancient) SR-5?
  • Tips for buying used electrostatic headphones? I believe that asking for sound balance is important.
 
Normal bias is obsolete, all the newer Stax (1990's and newer) use Pro bias.
L300 is pretty good for entry level. Can also try older 404 and 407 models.
Of course, you'll also need an energizer.