I have a Z830A. Similar except air cooled. I've been very happy with mine. Very rugged, cuts extremely well, fast, so far very reliable. I've owned it for about two and a half years and have just put filters and oil in it. That said, the 800s series transmissions are not as good as the newer 900 series. If the sticks are silver/grey it's a later model, better than the earlier ones, not as good as the 900s. If it has yellow sticks, it's an earlier model, I would avoid. My mower has about 600 hours, I haven't had any troubles with it yet, but the hydros were definitely the weak spot on those machines. I'm expecting to get somewhere between a thousand and 1500 hours before I have to rebuild/replace the transmissions. You'd definitely want to keep up with hydraulic fluid/filter changes to maximize the machine's life. Which unfortunately are a bit of a pain (the filters are underneath the machine and mount internally in the reservoir for each wheel's pump/motor circuit) and not cheap ($80+ total per change between two filters and hydraulic fluid).
