Others have given you a list of what to inspect, etc. If you buy it, just don't overload it (i.e., too much weight on the core aerator, too much weight in a cart, etc.) and expect it to last. I moved to an X530 from an X300. The difference is night and day for my use, but not everyone needs an X500-series to do their job.
I'm going to look at a used 2009 X300 tonight. It has been used to mow 3 arces and to push a snow blade in the winter. Machine looks mint in the pictures and seller claims the machine was babied, stored in doors from new, and always serviced at one of two dealers in the area. No paperwork on that though as she is selling due to divorce and and the papers are with hubby. The machine is her's though. (I hope)
The concern from what I've read is the K46 tranny. From what I read, towing loads and working the machine on hills will kill that tranny, I would imagine pushing snow would too although the cold ambient temperatures might save it. I suspect the failure is caused by heat right? 210 hours is a tad more than I wanted, I feel like the machine could go for years or die next week, I've read about machines dying at 100 hours, but then you have no idea what they were being used for.
Any pointers on what to inspect for and thoughts on the future longevity of that tranny would be much appreciated. For what it's worth, I'd expect to put 25 hours on it a year, cutting slighty sloping lawn....