What year MH models are too old to consider buying?

There is no "good" answer here. It depends. Depends on the particular MH in question, your skill with repairs, budget, and what you want. No one can tell you what to buy. MH's change over the years like anything else. There is no set timeframe when any particular MH will start having parts fail. Really depends on the MH and how it was cared for and maintained. I can look at 2 69 Camaros. one will be a heap of junk and one might be totally restored and updated.