Keep them greased, don't leave them with manure in them, Don't load them too heavy, you should be fine. Biggest issue to look for is the detachable link bed chains, if they are worn out they will break, if they break you get to shovel the bed out. Thats where not loading them too heavy comes into play. With mixed manure and bedding its not too heavy and you can load them right up, If its wet heavy stuff half a load is enough. I use a John Deere from the 40's, have for years. We had a New Idea model 12a for years but it didn't get used much and someone asked to buy it, they offered more than I thought it was worth. The JD is a little heavier and has more capacity thats why I prefer it. We spread 100 ton per year, 15 should be a breeze.