I sold my first Z, a 2002 Gravely 250 to a friend of mine with about 700 hours. He is still running it, with about 1200 hours, but has had to replace a starter, one spindle, and has had some electrical problems. I sold my second, a 2007 Gravely 260 with 930 hours to my daughter and her husband last fall. They mow about 1 1/2 hours a week. Only issue they had all this summer was it wouldn't start. Turned out to be just the safety switch on the parking brake wasn't making contact. My daughter fixed that herself.
By the way, the older Gravely's only have ONE grease zerk. The newer ones, none. With your situation, change the oil once a season, at the end, make sure you keep Sta-Bil in the gas all the time (because it will take you all summer to use full tanks), store it in the dry, and you should have minimal problems.
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