Welcome to the forum.
1. I'm not aware of specific issues with the Triton Studio. Things to look for are pretty standard across all synths. Take along a good pair of headphones with a 1/4" jack stereo connection or suitable adapter, and your favourite sustain pedal. Look at the general physical condition, scratches, evidence of being gigged, spilt liquids, that sort of thing. Check all of the keys work, and the key action is smooth, with no dropped or broken keys, and that the sound quality is good, without hum, crackles or pops. Check all of the controls work as expected. Some of the buttons may need to be pressed a little harder on a synth of this age, but that's normal.
2. $600 seems a fair price these days if all of the above test results are good-excellent. If anything, used synth prices seem to have risen over recent years, so it might even be considered an investment. Sticky keys, and not very responsive buttons can normally be fixed and, in any case, regular servicing is something that should be considered.
3. The more popular "bread and butter" sounds tend to be carried over from synth to synth, though they tend to evolve slightly along the evolutionary tree. Listen to the sound demos on e.g. YouTube e.g.
https://www.youtube.com/watch?v=VYrM_HGayOA
and look at the voice name list:
https://www.korg.com/us/support/downloa ... /219/1668/
There were a number of expansions available for the Triton Studio, each bringing new sounds to the synth. Check the start-up screen for info on any of these being fitted. Expansions would tend to increase the price, unsuprisingly.
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