There's a special ending when you do all dungeons and all memories. That's a pretty decent criteria (getting the special ending I mean).

The amiibo necessary for Epona isn't that expensive. Buying the Wii U version to run the game when you have the Switch version already because of loading times is. I don't see why we're griping about one amiibo here. If it turns out that we could beat the final boss with only the amiibo weapons that you can get (in which case you'd need several to be relevant), then yes it would be an issue, but I don't see that being a problem for a while.