It is best to buy new rather than vintage, and I agree that the Yamaha and metal tray Suzuki models are the best bets. But you might consider the Yamaha P-32D or Suzuki M-32C as well as the P-37D and M-37C. A 32 key model is easier to play but not inferior in sound. The Suzuki M-32C has not been available on Amazon or eBay lately, but the other three are on Amazon, with the Yamaha P-32D for about $70 including shipping. For a beginner (on the melodica) who wants a high quality melodica for a reasonable price, my recommendation would be the Yamaha P-32D (or P-37D if you have to have the extra five keys — I like the compactness, balance, and easy portability of the 32 key models.)
The distinction between beginner and advanced models is somewhat artificial. Almost any melodica, even one that’s cheaply constructed, will last many years, though you may not be satisfied with it for long.