This might be asking for trouble. Been learning cw for some time copy pretty good W1AW my curent radio is a marine ssb no cw. Would like to get started. Thinking second hand around $500. anybody got an opion on what might be the best second hand radio for cw only in this price range. I live on a boat so am limited in power and antenna size.
I'm no expert, but I'd recommend anything Ten-Tec for CW. A Scout (Ten-Tec 555) would work, or for home use an old Omni C (Ten-Tec 546C) -- if in good working condx -- is hard to beat for a great affordable CW xcvr. If you can spend a little more get a used Jupiter. It will give you all you need in a CW transceiver and also does SSB. Another option might be an Argonaut V (Ten-tec 516) (20 watts) -- does SSB too but they are almost as expensive used as they are when they were discontinued in 2008.
On the cheap side get a used Scout -- a solid little rig and plenty of power for CW (50 watts) -- you can choose the band modules you prefer. The Scout was also made to be a mobile rig so should fit nicely into your boat scenario if you don't mind a very basic older rig. The Argonaut V (516) was also designed as a mobile rig and has a lot more modern features than the Scout and is only 20 watts but will be hard to find for less than $600. (if you can find it for that). If you can afford a little more get a used Jupiter. I have the older version (green/gray screen) and it is everything I need on a xcvr (CW, SSB, 100 watts).
For pics and specs of the above mentioned rigs, check out Ten-Tec wiki at: http://tentecwiki.org/doku.php?id=transceivers
just click on a model # to go to that models info sheet
Elecraft and Ten-Tec are what most CW devotees tend to prefer...