Are you using a phone you have updated before or was it a phone with windows 8.1 with all you apps and contacts etc?
I had a problem with no store and many other programs. I tried many things and read like you and found nothing. In frustration I reset the phone in settings/About. The phone restarted and the cogs labored away for 10-15 mins and then the screen said it was updating again and all of a sudden all the apps and everything worked.
I think it has something to do with conflicting with existing settings and apps.
This was the error message I received for reference.
0x803F8001