I see a lot of super z's with 600-900 hours on craigslist for 3500-4000. I wouldn't buy any gas ztr with 1500 hrs on it. If the person didn't do regular maintenance or put it through heck it is about done in most cases. Kubota diesel should go close to 4000 hrs or more even with abuse. These findings are from experience with mowers we use. Have 2 kubota diesels that are around 2000 hours and 2 deeres 820a and 810 a both with kawasaki's. One motor went out at 1500 and the dealer said that was not unusual for them to go out between 1500-1800 hrs. We do use ours pretty rough though. THere are some jobs where we do 3 miles of steep ditches in some developments.
With the deals that are going on now though your best bet is to buy new with warranty. I myself am going to buy the z915b at 7000 this month with 3k down my payments will be 119 a month for 36 months.