here is a link too check out specs and reader reviews,etc,etc.
http://www.sonicstate.com/synth/_inc/type.cfm?type=works
also go to ebay and check out the instruments...and in the instrument category you will find used synths in the synth section and electronic keyboard section.....I seen
an ensoniq asr10 rack under 400 thats a really hot piece right there...dont let the age fool you...kanye,notzz,just to name a few still use them.....If this interests you
the ensoniq eps,eps16plus are the older version of the ASR10 witch you can get same quality of sound.
my advice is browse ebay for various synths keyboards..if something catches your eye then hit this link
http://www.sonicstate.com/synth/
has all the manufactures of synths and you can get all the info and reader reviews..wich is important,cause when you go to the makers page they can make anything sound good...take your time and dont get anything without comparing....Ive wasted alot of money by trial and error...also an older piece to consider is
a Roland W30.
I think at this point and your experience level any older piece is a no lose situation.Not only are you gonna learn all the functions of that synth wich can also be applied to any synth you get after that,but the older keyboard has depreciated as much as its going to at least for a period of time...Buy a new$1800 keyboard today and in 2 years it will go for about $1000,so if you end up not liking your used keyboard you most likely wont lose...Dont judge a book by its cover....You can read the specs of a new..lets say sampler with nearly unlimited sampling time and an older one only had about 2 seconds of sampling..on paper the new one kicks its butt,but in application the older one sounds better..I had an E.M.U.
sp12turbo originally
the sp12 went for around $2700 and to make it a turbo wich made it go from 1.2 seconds of sampling to 7 seconds cost an additional $800 as an upgrade......7 seconds of sample time aint nothing...but people still buy them today