Do you think Amazon.com is a good site to do so...
I think Amazon is a good place to buy, but I think that Google Play is the best by far because it's the official, and probably you'll get the updates early.
And you'll be able to use some features like Google Play achievements for Games. And I hope that GPlay get better in the backup issue, and when it happens you'll get support for this.
I just buy anything outside the GPlay if there is no alternative to download an App that I like too much.