Check out http://www.vintagesynth.com/
and use the synthfinder (prepare for your mind to be blown)
VintageSynth has reviews on just about everything you will be able to find used. I never buy new for the exact reason mentioned above. Keyboard technology changes crazy fast so $5000 keyboards from the 80s can be found for $200 on eBay. Things that are out of fashion, like 90s digital keyboards can be found cheaper. Some keyboards are iconic or rare and hold their value a bit better.
Another option would be a sound module. These have largely been replaced by laptops in recent years, but they are essentially the guts of the keyboard without the keys. You can play them onstage with a controller using MIDI. If you find a keyboard you like but it doesn't have all of the sounds you want, then you could add a sound module for more options.
Good luck and let us know what you decide on.