> How can you know which phone to pick that should get updates for at least 2 years if not longer?

Pretty much impossible to predict. The safest bet is to either buy a Nexus phone (Nexus 4, 5, etc.) or maybe the GPE of a phone (S3, One, Moto X). Google stopped supporting the Galaxy Nexus so people were a bit miffed about that.

The good thing with Android is that even if your manufacturer/carrier decides to stop supporting your phone, you still have the opportunity to flash an updated ROM via third-party developers. Granted the performance probably won't be the best but at least you still have the option as opposed to iOS.