I'm with Farmer Z. I run nothing but older equipment around here, and won't consider replacing it unless the reliability is truly getting poor. I've got an 1135 Massey thats broke down more than its running, so it gets one more chance and then it goes away. The only reason it gets one more chance is because its always something different breaking down, and its normal wear items, albeit pricy ones. I've also got an 1130 Massey that gives me minor grief about once a year, so it'll stay around for quite awhile. About the only other thing I'd consider is the cost of repair. If its going to cost more to fix it than I can buy another older replacement for, it'll see the bush or head off to the wrecker.
Rod