If you don't want to race, then I doubt you need anything with internal engine work. Standard pipe / FI mapping should be fine. At that standard I don't think your price is unreasonable. No one who puts suspension and track plastic (and exhaust and FI computer) on a track bike is going to get their money back out of it again. It just doesn't happen. Good stuff in the 6k range comes up for sale all the time, and 09 is already 5 years old...but you may have to look at slightly older machines to keep the price in that range. Speaking generally (not about ZX6's which I know nothing specifically about), moderate suspension mods = $2000, pipe and map = $1000, plastic = $500 net of sale of stock parts, paint = $100 to $800 depending. Reasonably, divide that total in half as an estimate of what a reseller can hope to get back, and that is about $2000 in mods on top of the value of the bike. If it has rearsets, quickshifter etc. that stuff is worth paying for as well, but again at no more that 50% of cost new.
Sorry, I don't know about a good online market for such things. The old cherrypicker racing site was excellent, but is defunct. Some stuff comes up on kijiji but searching for it is hard. Word of mouth may be the best. People sometimes post track bikes for sale here on GTAM.