There's a lot to like with AppEngine, but the support is showstopper for anything you can't afford to be down.

Outbound emails, for example, mysteriously quit working for a month straight until someone there admitted that the anti-spam dial was tuned incorrectly. There was no error back to the users indicating the emails were being trashed. https://code.google.com/p/googleappengine/issues/detail?id=1...

You might check out the various lower cost plans that many providers offer to startups. Here's a few:

https://www.digitalocean.com/hatch/

https://cloud.google.com/developers/startups/

https://azure.microsoft.com/en-us/pricing/member-offers/bizs...

https://aws.amazon.com/startups/